docs(ai): partition ai.md into five subsystem files (verbatim moves)

ai.md 1,308 → 228 lines. No prose rewritten: every block was moved as-is so
nothing could be lost in translation, then replaced by a pointer.

New files:
- docs/ai/environment.md — LAN/SSH + Gitea push recipe, Windows dotnet
  build/run, test-suite map, hermetic real-MainWindow test rule
- docs/ai/product.md — brand/naming, AI transparency, GPL licensing
  prohibitions, free-forever monetization, auth/account assumptions,
  packaging & distribution
- docs/ai/youtube-api.md — go-live ordering, the broadcast-binding fix, the
  private-only lock, health parsing, end-stream pre-flight, Test mode,
  chat insert shape
- docs/ai/overlays.md — social bar, native alert box (animations, clip,
  ticker), chat box conventions
- docs/ai/ui-surface.md — control surface, scene strip, preview rail,
  hotkeys, broadcast drawer, toasts, transitions, brand logo, launch
  geometry, theme/properties conventions

ai.md keeps the response/derivation rules, the architecture table, the four
existing invariant stubs, and one pointer per moved subsystem.
docs/ai/index.md routes every file with a "read when" column.

References falsified by the move were fixed in the same commit: AGENTS.md
build note → environment.md → Run, and the "Related" footers in
frame-pipeline.md / output.md / capture.md now point at the owning files.

Docs-only unit: scope-check run, no build (no source touched).

docs/ai/frame-pipeline.md

@@ -122,7 +122,7 @@ allowed until a real reading names the culprit.
 
 **Read takes in Release.** The creator launches `dotnet run -c Release`; Debug pixel paths are
 several× slower, so a Debug number is not a reading of this pipeline (see
-[`ai.md` → Run](../../ai.md#run)).
+[`environment.md` → Run](environment.md#run)).
 
 **Derivative check (AGENTS.md):** this is not invented — OBS's own answer to "which source is slow" is
 the **Source Profiler**, which reports the *average execution time of each source's tick and render
@@ -793,11 +793,11 @@ Full suite 290/291 passing, the sole failure the pre-existing compositor pixel t
 
 ---
 
-## Related in [`ai.md`](../../ai.md)
+## Related files
 
-This file owns the compositor and the pump. The neighbours stay in `ai.md`:
+This file owns the compositor and the pump. The neighbours live elsewhere:
 
-- **Live encoder + RTMP push** — `IFfmpegEncoder`/`FfmpegEncoder`, `FfmpegArgs`, locator and bundle.
-- **Media source** — `MediaVideoSource` (ffmpeg `rawvideo` decode) feeding the pump's resolver.
-- **Social bar** and **Native events & alerts** — the overlay strips the compositor blits last.
-- **Run** and **Tests** — how to launch in Release and how the suite is run.
+- [`output.md`](output.md) — **live encoder + RTMP push**, `IFfmpegEncoder`/`FfmpegEncoder`, `FfmpegArgs`, locator and bundle.
+- [`TASKS/task-21-media-source.md`](../../TASKS/task-21-media-source.md) — **media source**, `MediaVideoSource` (ffmpeg `rawvideo` decode) feeding the pump's resolver.
+- [`overlays.md`](overlays.md) — **social bar** and **native events & alerts**, the overlay strips the compositor blits last.
+- [`environment.md`](environment.md) — **Run** and **Tests**, how to launch in Release and how the suite is run.