docs(toolchain): update flow for all three skill sets

The 'To update' note only covered the Blender packs. Add the Unreal Engine and
C++ sources, each with its git source and the commit to record in
skills.manifest.json, and note that the manifest is the install record to keep in
step with the table.

Docs/toolchain.md

@@ -41,7 +41,13 @@ Unreal Engine 5.9 is in preview and Unreal Engine 6 has been announced as of Nov
 
 All skill packs — Blender, Unreal and C++ — are agent instructions, not code that runs in a build, and **none are committed**: `/.agents/skills/` is gitignored and the Blender packs live in the common `~/.pi/agent/skills/` root. Four of the Blender packs assume a **Blender MCP server**; this project drives Blender headlessly through `bpy` (`Tools/*.py`) instead, so MCP is optional and nothing depends on it.
 
-To update: copy `.claude/skills/` from a newer upstream checkout over the local directory, then record the new version and commit hash in this table. Skills are read on demand, so the harness catalog may need a reload before they appear as first-class entries.
+To update a pack:
+
+- **Blender** — copy `.claude/skills/` from a newer `arjun988/blender-skills` checkout over `~/.pi/agent/skills/`, then record the version and commit in the table above.
+- **Unreal Engine** — re-install the `quodsoler/unreal-engine-skills` git source at the new commit (`pi install <source-url>@<commit>`); the 31 packs land under `.agents/skills/ue-*/`. Record the commit in `skills.manifest.json` and the table.
+- **C++** — re-install the `DiamonDinoia/skill-cpp` git source at the new commit; the router lands at `.agents/skills/cpp/`. Record the commit in `skills.manifest.json` and the table.
+
+`skills.manifest.json` (in `.agents/skills/`) is the install record — the git sources, commits and layout — and is the thing to keep in step with the table. Skills are read on demand, so the harness catalog may need a reload before they appear as first-class entries.
 
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