engine(macos-arm64): 4.7 native-module build + ship GodotSharp; add update-engine skill

- Replace macOS binary with the 4.7 build where Terrain3D + agent_skinned are native
  engine modules (no GDExtension .framework). Fixed C# bindability (set_plugin->Object,
  abstract Terrain3DAssetResource, drop dup duplicate bind) so glue generates.
- SHIP GodotSharp alongside the binary (matched pair) — stale/missing GodotSharp causes
  "Wrapper class not found for Terrain3D" / ".NET assemblies not found". Engine does not
  regenerate it. Un-ignore GodotSharp in .gitignore.
- manifest.json: macOS -> 4.7 (4ba7b5b) + new sha + godotsharp field; drop obsolete
  terrain_3d GDExtension section; windows flagged 4.7-pending.
- Add .agents/skills/update-engine (mac+win self-serve install) + README + Forge handoff.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -1,11 +1,40 @@
 # tinqs/builds
 
-Build artifacts for the Tinqs stack — engine editor binaries and game exports.
-**Binaries are stored via Git LFS** (see `.gitattributes`). Source lives in `tinqs/engine`,
-`tinqs/ariki-game`, `tinqs/ariki-sim`. See `manifest.json` for what's here.
+Build artifacts for the Tinqs stack — engine editor binaries (+ their GodotSharp C# bindings).
+**Binaries and GodotSharp are stored via Git LFS** (see `.gitattributes`). Source lives in
+`tinqs/engine`, `tinqs/ariki-game`, `tinqs/ariki-sim`. See `manifest.json` for exact versions.
 
-Long-term plan: CI (Gitea Actions) builds on push and publishes here / to S3 — see
-`docs/ai/plans/build-pipeline-architecture-2026-06-07.md`.
+> **Use `tinqs pull` / `tinqs push`, never raw git** — these are LFS-heavy.
+
+## Installing / updating the engine
+
+**Don't copy files by hand — run the skill:** [`.agents/skills/update-engine`](.agents/skills/update-engine/SKILL.md).
+Hand it to your agent; it installs the binary + GodotSharp into `../engine/bin/`, wipes stale
+caches, and verifies. Covers macOS + Windows.
+
+## ⚠️ The one rule: binary + GodotSharp are a MATCHED PAIR
+
+Each engine binary ships with a `GodotSharp/` folder (Godot's C# bindings) **generated from that
+exact engine build**. You must install them **together** and replace any old `GodotSharp`.
+
+- The engine does **NOT** regenerate GodotSharp at runtime. If it's missing you get a
+  **`.NET assemblies not found`** dialog pointing at `engine/bin/GodotSharp/Api/Debug`.
+- A **stale** GodotSharp (from an older engine) against a newer binary throws
+  **`Wrapper class not found for type: Terrain3D`** → terrain renders as water, animals don't animate.
+
+This matters now because **Terrain3D and `agent_skinned` are native engine modules** (no separate
+`addons/terrain_3d` GDExtension `.framework`). Built-in module classes need their wrappers baked
+into GodotSharp, so GodotSharp had to be regenerated — and must ship here alongside the binary.
 
 ## Current contents
-- `engine/macos-arm64/tinqs.macos.editor.arm64.mono` — Godot 4.6.3 fork editor (mono), built on Kraken with Xcode 26.3.
+
+| Platform | Engine | Terrain3D | GodotSharp shipped? |
+|---|---|---|---|
+| `engine/macos-arm64/` | **4.7** (`4ba7b5b`) — native modules | built-in module | ✅ yes (`GodotSharp/`) |
+| `engine/windows-x64/`  | **4.6.5** (`420e74bf8`) — still GDExtension | GDExtension | ❌ pending — see handoff |
+
+**Windows is not yet on 4.7.** The 4.7 Windows build + its GodotSharp must come from Forge —
+see [`.agents/handoffs/forge-windows-4.7.md`](.agents/handoffs/forge-windows-4.7.md).
+
+Long-term: CI (Gitea Actions) builds on push and publishes here / to S3 —
+`docs/ai/plans/build-pipeline-architecture-2026-06-07.md`.