feat(extensions): version-control pi mempalace extension + install.sh symlink

The pi coding-agent extension at ~/.pi/agent/extensions/mempalace.ts was
living only on tor-ms22, including hand-edited fixes (Type.Unsafe
schema-passthrough for MCP tool parameters). One disk wipe away from
losing it, and no way to reproduce the install on a new machine.

- extensions/pi/mempalace.ts: canonical copy (matches tor-ms22 byte-for-byte)
- extensions/pi/README.md: what it does, the schema-passthrough gotcha,
  debugging knobs
- install.sh: new install_pi_extension step — gated on ~/.pi/agent/extensions/
  existing, backs up any real file in the way, idempotent re-runs, mirror
  block in uninstall. Works on macOS and Linux (plain ln -s, readlink -f).
- README.md: mention extensions/pi/ in the repo-contents list and in the
  Setup section

Verified on tor-ms22: install (backs up existing real file) → uninstall
(removes symlink) → reinstall (clean symlink). Re-runs are no-ops.

README.md

@@ -8,6 +8,7 @@ Producer-side tooling for [MemPalace](https://github.com/MemPalace/mempalace)
 - `bin/mempalace-docs` — mines project directories into MemPalace while excluding source code, keeping the palace signal-dense.
 - [`ARCHITECTURE.md`](ARCHITECTURE.md) — **canonical spec**: architecture diagram, component details, setup recipe, operational notes, upstream-retirement roadmap.
 - [`SKILL.md`](SKILL.md) — the companion agent skill, symlinked into `~/.agents/skills/opencode-mempalace-bridge/` on install.
+- [`extensions/pi/`](extensions/pi/) — the [pi coding-agent](https://github.com/mariozechner/pi-coding-agent) extension that bridges pi to the MemPalace MCP server (wake-up auto-injection, `/mempalace-diary` command, schema passthrough). Symlinked into `~/.pi/agent/extensions/` on install when pi is detected.
 
 **If you're just trying to get this working on a new machine → jump to [Setup](#setup).**
 **If you want the full architecture story → read [`ARCHITECTURE.md`](ARCHITECTURE.md).**
@@ -221,7 +222,7 @@ cd ~/mempalace-toolkit
 ./install.sh
 ```
 
-The installer symlinks `bin/*` into `~/.local/bin/` and optionally installs the agent skill into `~/.agents/skills/opencode-mempalace-bridge/`.
+The installer symlinks `bin/*` into `~/.local/bin/` and installs the agent skill into `~/.agents/skills/opencode-mempalace-bridge/`. If [pi](https://github.com/mariozechner/pi-coding-agent) is installed (detected via `~/.pi/agent/extensions/`), it also symlinks [`extensions/pi/mempalace.ts`](extensions/pi/) into that directory so the pi↔mempalace bridge tracks version control. On machines without pi this step is silently skipped. Works on macOS and Linux.
 
 Ensure `~/.local/bin` is on `$PATH`: