chore(experiments): add registry of existing experiments - #30919

Adds experiments/registry.yaml, an inventory of every experiment on main today. Nothing reads it yet. It is the first piece of the experiment promotion gate: a later PR adds a lint check that keeps it in sync with the code and enforces the stage rules.

The scan covered every marker the repo uses: the 17 flags in codersdk.ExperimentsKnown, the x/ directories, exp_ files, /api/experimental routes, docs/manifest stage labels, and the frontend experiments.includes(...) gates. Each entry records the stage, the date it entered that stage, the flag, the paths it owns, its experimental routes and its docs page. A tools section lists exp_ files that are dev tools, not features (scale tests, debug commands).

27 entries: 17 flagged experiments, 10 unflagged ones. Stage comes from the docs label. A feature behind a hidden flag with no label is early access.

Drift the scan found, recorded in each entry's notes:

• Coder Agents and agent skills are GA, but their code is still under x/ and some routes are still under /api/experimental.
• mcp-server-http: the docs label the page beta, but the endpoint needs a hidden flag.
• notifications: nothing reads the flag since chore: remove notifications experiment #14869, but it is still a known experiment.
• chat-virtual-desktop: the desktop stream route and the agent desktop API are not gated by the flag.
• Virtual desktop and debug-logging docs name /api/v2 routes that do not exist.
• startup-coordination and external-workspaces are early access by docs label only. No flag gates them.

Left out: JetBrains Toolbox (labelled beta, but no gate or experimental code) and the removed agents and chat-automations flags. No owner field yet, since there is no team mapping to fill it.

Testing

A local script (not committed) checked that every path glob matches at least one tracked file, every docs page exists, the flags match ExperimentsKnown exactly, and every x/ and exp_ file belongs to an entry or a tool. Six routes are missing from swagger; each was confirmed in the router.

Xum acted for @ibetitsmike to open this PR.