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/capture.md

@@ -476,9 +476,7 @@ new panel must use a different binding (`IsWebcamSelected`) or the selector matc
 
 ---
 
-## Related in [`ai.md`](../../ai.md)
+## Related files
 
-- **Resolution tiers** — `OutputRect*` and the tier list (the vertical tier's 607×1080 band).
-- **Key patterns** — MVVM/binding conventions the capture UI follows.
-- **Tests** — hermetic-real-window rules that camera tests must obey.
-- **Run** — launch/take procedure and the Release-only rule for measurements.
+- [`ui-surface.md`](ui-surface.md) — **resolution tiers** (`OutputRect*`, the tier list, the vertical tier's 607×1080 band) and **key patterns** (MVVM/binding conventions the capture UI follows).
+- [`environment.md`](environment.md) — **Tests** (hermetic real-window rules camera tests must obey) and **Run** (launch/take procedure, the Release-only rule for measurements).