skill: co-locate the pi-extensions agent skill in the package

Add skill/SKILL.md (+ skill/evaluate-extension-usage.py, referenced by the
skill via ./) so the canonical 'how to use fork/recall/ssh-controlmaster'
skill lives next to the extensions it documents — the single source of truth.

Motivation: the global AGENTS.md (pi-toolkit) tells every pi session to read
~/.agents/skills/pi-extensions/SKILL.md at session start to fix fork/recall
under-utilisation, but that skill previously lived ONLY in the private
skillset repo. In any environment without the skillset mounted (e.g. a
pi-devbox container started without it) the pointer dangled. Co-locating the
skill here gives a public, package-owned source that downstreams can vendor.

install.sh is intentionally unchanged: skill deployment on a normal
workstation stays the skillset repo's responsibility (no double-deploy).

README.md

@@ -19,6 +19,17 @@ chmod +x install.sh
 
 Each `.ts` file in `extensions/` is symlinked into `~/.pi/agent/extensions/`. Existing real files are backed up with a timestamp. Re-runs are idempotent.
 
+## Bundled skill
+
+`skill/SKILL.md` is the agent skill that documents *how to use* these extensions
+(fork tier selection, recall discipline, task design, remote-pi mechanics). It
+lives here, co-located with the code it describes, so it travels with the
+package and can be vendored by downstream consumers (e.g. `pi-devbox` bakes it
+as a fallback skill so containers have it even without the `skillset` repo
+mounted). `skill/evaluate-extension-usage.py` is referenced by the skill and
+must stay alongside it. `install.sh` does not deploy the skill — skill
+deployment remains the `skillset` repo's job on a normal workstation.
+
 **Install a subset:**
 
 ```bash