feat(tui): hidden experiments section with per-tab prompt drafts - #41862

What

Adds a hidden Experiments surface to the TUI: an opt-in home for in-flight features on any channel, reachable only by typing /experiments (it is deliberately absent from the ctrl+p command palette). Each experiment is a registry entry with a title and description; toggling writes experimental.<id> to the TUI config. Graduating or killing an experiment means deleting one registry entry plus its gated branch.

Ships one experiment to give the surface a real tenant:

Per-tab prompt drafts (tab_drafts) — today an unsent prompt draft is a single global slot that follows focus across session tabs. With the experiment on, drafts stay on the tab where they were written, and <leader>n (new session) moves the current draft into the new session instead of leaving a copy behind.

1. Draft on tab two

[Session one]  [Session two]
                active
> WIP draft: notes for session two_

2. Switch to tab one — draft stays behind

[Session one]  [Session two]
 active
> _

3. Switch back — draft restored

[Session one]  [Session two]
                active
> WIP draft: notes for session two_

4. <leader>n — draft moves to the new session

[Session one]  [Session two]  [New session]
                               active
> WIP draft: notes for session two_

Tab two's composer is now empty (moved, not copied).

How

• packages/tui/src/config/index.tsx — adds an experimental struct to the TUI config schema (experimental.tab_drafts).
• packages/tui/src/component/dialog-experiments.tsx — new experiment registry + DialogExperiments (a DialogSelect that toggles experimental.<id> via config.update).
• packages/tui/src/app.tsx — registers opencode.experiments with palette: undefined + slash: { name: "experiments" }, so it is typeable but not listed; session.new moves the in-progress draft into the home route's prompt seed when the experiment is on.
• packages/tui/src/component/prompt/index.tsx — the module-level draft stash gains a Map keyed by tab (sessionID, or home) when the experiment is on. Two subtleties:
  • the session ID is captured once at Prompt setup; reading props.sessionID lazily would observe the next route during onCleanup and stash the draft under the wrong tab (the draft would follow you, one tab behind)
  • the carried draft is shallow-copied before reset(), because reset() merges an empty prompt into the same underlying store object that unwrap exposes

Default behavior (experiment off) is byte-identical to today: one global stash slot.

Scope

• No experiments UI in /settings — the section is intentionally separate and unlisted. Happy to surface it differently if we want more/less discoverability.
• The stash map is unbounded across closed tabs (entries are consumed on restore; abandoned drafts of closed sessions linger in memory for the process lifetime). Trivial to cap if we care.
• No tests yet — the stash is module-level state inside the Prompt component; testing it well probably wants a small fixture harness. Will add if we like the direction.

Testing

• bun typecheck in packages/tui — clean.
• End-to-end via opencode-drive against this branch (--dev): created two sessions, enabled the experiment through /experiments, verified the draft stays on its tab, restores on return, moves on <leader>n, and the source tab's composer is empty afterwards. Also verified experimental.tab_drafts: true lands in the config file.

Demo

Real end-to-end run recorded with opencode-drive (simulated model, real TUI from this branch): creates two sessions, opens /experiments, toggles the experiment on, writes a draft on tab two, clicks between tabs to show the draft staying put, then ctrl+x n moving it into a new session.