#27246)

## Summary

Phase 2 of the H1 → front-matter migration (`DOCS-483`; parent
`DOCS-477`). Makes the two reference-doc generators emit per-page
metadata as YAML front matter instead of a leading `# H1`, so generated
pages are self-describing and `make gen` stops reverting migrated pages
(Phase 3).

Phase 1 (`DOCS-482`) made the coder.com renderers prefer a front-matter
`title` (manifest fallback).

> [!NOTE]
> Rebased onto `main` and fully regenerated, and updated across two
rounds of Coder Agents Review — see **Review follow-ups** below.

## Changes

- **`scripts/clidocgen/command.tpl` + `gen.go` + `main.go`** — front
matter now carries `title` (from `fullName`) and `description` (from the
command's `Short`), and the leading `# H1` is dropped. The CLI index
page's front matter is taken from the manifest `Command Line` route
(title/description/icon_path).
- **`scripts/apidocgen/postprocess/main.go`** — reads the manifest and,
at write time, injects front matter carrying each section's `title` plus
any curated `description`, `state`, and `icon_path`. The API index
page's front matter is taken from the manifest `REST API` route.
- **`scripts/docgenenv`** (new shared code) — one `YAMLScalar`
front-matter escaper, one `Route`/`Manifest` schema +
`LoadManifest`/`FindRoute`, and one `FrontMatter(Route)` emitter, all
imported by both generators (no duplicated helpers, types, or emitters).
- Regenerated all **166 CLI + 31 API** reference pages.

### Metadata → front matter, and what stays in the manifest

Every *per-page* manifest field is mirrored into the page's front
matter: `title`, `description`, `state`, `icon_path`. The **structural**
fields stay in `manifest.json`:

- `children` — the nav tree (explicitly out of scope).
- `path` — the manifest's pointer to the file; a page carrying its own
path is redundant/error-prone, so it's treated like `children`.

The fields are **duplicated** into front matter and **`manifest.json` is
left unchanged**, so this is a **no-op for rendering today** (coder.com
strips front matter for `llms`, and Algolia + the renderer read only
`title`). Removing the fields from the manifest is the natural
follow-up, gated on the renderer reading them from front matter first.

### Why the API side changes the postprocessor, not the `.dot` templates

The issue text suggested editing
`scripts/apidocgen/markdown-template/*`. I deliberately did **not**,
because the postprocessor derives each page's **filename, section title,
and manifest route** from the leading `# {name}` line
(`extractSectionName`). Emitting front matter from the template would
break that extraction. Instead the widdershins templates still emit `#
{name}`, the postprocessor reads it (and now verifies it), and then
swaps the heading for a front-matter block as each section is written.

## Review follow-ups (Coder Agents Review)

### Round 1 — addressed in `e53d5e03` (all threads resolved)

- **CRF-1 / CRF-4** — de-duplicated the escaper and the
`route`/`manifest` schema + traversal into `scripts/docgenenv` (shared
by both generators).
- **CRF-2** — `YAMLScalar` now quotes YAML-reserved scalars
(`true/false/null/…`, numbers); no current value is affected.
- **CRF-3** — added unit tests: a `YAMLScalar` round-trip, `FindRoute`,
and `prependFrontMatter`.
- **CRF-5** — the CLI and API **index** pages now mirror their manifest
route's title/description/icon_path instead of a hardcoded
`coder`/`API`, fixing a rendered-heading regression (`REST API`/`Command
Line` were being overwritten).
- **CRF-6** — dropped the dead `#login` anchor in
`docs/support/support-bundle.md` (the migrated `login.md` no longer
mints that heading anchor).
- **CRF-7 / CRF-8 / CRF-11** — renamed to `prependFrontMatter`, switched
to `bytes.Cut`, and it now strips the first line only when it is the `#
{name}` heading (`extractSectionName` errors otherwise).
- **CRF-9** — removed the orphan `docs/reference/api/chat.md` (not in
the manifest, not linked; the real page is `chats.md`).
- **CRF-10** — the metadata read and the manifest rewrite now share one
`FindRoute` traversal.
- **CRF-13** — moot under squash-merge; this branch is a single
scopeless commit.
- **CRF-15** — the pre-existing `sort.Slice`/`slices.IsSorted`
comparator is left as-is per the review (out of scope; safe today
because section names are unique).

### Round 2 — addressed in `ee796e7107` (all threads resolved)

- **CRF-16** (P1) — removed three em-dashes from new doc comments (the
only `make lint` failure on the prior head); the emdash gate is green.
- **CRF-17 / CRF-18** — unified front-matter emission into one shared
`docgenenv.FrontMatter(Route)`, used by the API postprocessor directly
and by `command.tpl` via a `frontMatter` template func. This retires the
hand-written template YAML and the
`indexTitle`/`indexDescription`/`indexIconPath` closures, so a new
front-matter field is wired in one place, and it gives the CLI index the
`state` arm it previously lacked. Verified byte-identical: a full CLI +
API regen produces zero page changes.
- **CRF-19** — CLI child sort switched to `slices.SortFunc` +
`cmp.Compare` (typed comparator).
- **CRF-20** — reworded the `prependFrontMatter` comment:
`extractSectionName`'s fail-fast is the load-bearing guard; the prefix
check is a defensive backstop.
- **CRF-21** — added `icon_path`/`state` coverage in `docgenenv`'s
`TestFrontMatter/AllFields` (the branch the index page relies on,
previously at 0%).
- **CRF-22** — `YAMLScalar` no longer emits a trailing-space value as a
bare scalar (YAML strips it on read, so it would not round-trip); added
test coverage.
- **CRF-24** — the shared emitter removed the duplicated `cliIndexRoute`
doc comment; the rationale now lives in one place.
- **CRF-23** (Phase 3, out of scope here) — noted: the API generator
wipes and regenerates `reference/api/` from the manifest, so removing
curated metadata from the manifest in Phase 3 needs another source first
(a generator that preserves existing front matter, or metadata carried
alongside the swagger annotations).
- **Process (Mafu-san)** — the verification set below now leads with
`make lint`, the mandatory CI gate that the earlier list omitted.

## Cross-repo dependency

**Resolved — this PR no longer has a hard merge-ordering gate** (CRF-14
was right; the earlier "must merge after #968" note was stale).

The coder.com surfaces that would otherwise leak raw front matter from
`coder/coder` `main` are already front-matter-aware on merged PRs:

- **coder.com#964** (`DOCS-554`, llms-full.txt corpus + Algolia) —
**merged**.
- **coder.com#974** (`DOCS-574`, the `.md` proxy twin + `llms.txt` index
titles) — **merged**.

coder.com#968 (`DOCS-577`) was re-scoped to only the renderer
route-metadata generalization; it's a no-op on today's corpus and its
own description confirms the "deploy before the generators" constraint
no longer applies (that was driven by the llms corpus, now in #964).
Worth a final confirmation that #964/#974 are **deployed** before merge,
but there's no branch/PR ordering blocker left.

## Verification & evidence

AI was the primary author of this PR (see disclosure below); per the [AI
Contribution
Guidelines](https://coder.com/docs/about/contributing/AI_CONTRIBUTING)
here is manual verification.

- `make lint` (golangci-lint + the emdash gate) passes; `go build` / `go
vet` / `go test` are clean for the generators + `scripts/docgenenv`;
`pnpm check-docs` passes.
- `swagger.json`, `docs.go`, and `manifest.json` are **unchanged** —
metadata is duplicated into front matter; command/section names and
routes did not move.
- The diff is purely additive front matter
(`title`/`description`/`state`/`icon_path`) + the leading H1 removal; no
body reflow. A full CLI + API regen produces **zero** page changes
beyond the two index pages.

<details>
<summary>Terminal evidence</summary>

CLI `description` from the command's `Short` (`YAMLScalar` quotes when
needed, e.g. a `Short` with a colon):

```md
---
title: server
description: Start a Coder server
---
```

API pages inherit curated manifest metadata (only Agents/Chats have any
today):

```md
---
title: Chats
description: "REST endpoints for Coder Agents Chats API (programmatic agent sessions)."
state:
  - early access
---
```

Diff scope + "no body changes" proof (uses an explicit `base..HEAD`
range, so it actually tests the claim):

```
$ git diff --shortstat origin/main
 210 files changed, 1447 insertions(+), 344 deletions(-)
# = 166 CLI + 31 API reference pages + generators + scripts/docgenenv
# swagger.json / docs.go / manifest.json: NOT modified

# Every removed line under docs/reference is a leading "# H1"; nothing else:
$ git diff origin/main..HEAD -- docs/reference/ | grep '^-' | grep -v '^---' | grep -v '^-# '
(empty)

$ pnpm check-docs
Summary: 0 error(s)
```

</details>

Linear: DOCS-483

> This PR was created with AI assistance (Coder Agents).