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).

AGENTS.md

@@ -128,7 +128,7 @@ it or revert the change. The script enforces what the rule demands.
 From WSL, ALWAYS use the Windows dotnet host — Linux `dotnet` re-downloads the
 `windowsdesktop.app.*` packs over the slow 9p bridge and re-restores twice (WPF
 `_wpftmp`), and `--no-restore` right after an interrupted restore produces bogus
-`NETSDK1064` errors. See `ai.md` → Run for the exact commands and why.
+`NETSDK1064` errors. See `docs/ai/environment.md` → Run for the exact commands and why.
 
 ```bash
 "/mnt/c/Program Files/dotnet/dotnet.exe" build "C:\Users\gramp\Documents\Code\projects\ytLive\ytLive.csproj"