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.

install.sh

@@ -16,6 +16,11 @@ SKILL_SRC="${SCRIPT_DIR}/SKILL.md"
 SKILL_DEST_DIR="${HOME}/.agents/skills/opencode-mempalace-bridge"
 SKILL_DEST="${SKILL_DEST_DIR}/SKILL.md"
 
+# pi coding-agent extension (optional — only linked if pi is installed)
+PI_EXT_SRC="${SCRIPT_DIR}/extensions/pi/mempalace.ts"
+PI_EXT_DEST_DIR="${HOME}/.pi/agent/extensions"
+PI_EXT_DEST="${PI_EXT_DEST_DIR}/mempalace.ts"
+
 # ── args ─────────────────────────────────────────────
 ACTION="install"
 ASSUME_YES="no"
@@ -38,12 +43,15 @@ What install does:
   - Symlinks SKILL.md into ~/.agents/skills/opencode-mempalace-bridge/SKILL.md
     (auto-discovered by opencode; run agents-sync from cli_utils to also
     reach Claude Code and Kiro)
+  - If pi (~/.pi/agent/extensions/) exists, symlinks extensions/pi/mempalace.ts
+    into ~/.pi/agent/extensions/mempalace.ts (pi bridge). Skipped otherwise.
   - Drops a .skill-source marker in the skill dir so sibling tooling
     (deploy-skills.sh, agents-sync.zsh) knows the dir is externally owned
 
 What uninstall does:
   - Removes symlinks in ~/.local/bin/ that point into this repo
   - Removes the skill symlink if it points into this repo
+  - Removes the pi extension symlink if it points into this repo
   - Removes the .skill-source marker and empty skill dir
 EOF
             exit 0 ;;
@@ -219,6 +227,36 @@ check_opencode_mcp() {
     return 0
 }
 
+install_pi_extension() {
+    # The pi coding-agent extension is optional: link it only if pi is
+    # already installed on this machine (its ~/.pi/agent/extensions/
+    # directory exists). Otherwise silently skip — mempalace-toolkit is
+    # useful on opencode-only boxes too.
+    if [[ ! -d "$PI_EXT_DEST_DIR" ]]; then
+        note "pi not detected at $PI_EXT_DEST_DIR — skipping pi extension"
+        printf '       (install pi first if you want the pi↔mempalace bridge:\n'
+        printf '        https://github.com/mariozechner/pi-coding-agent)\n'
+        return 0
+    fi
+
+    note "Linking pi extension into $PI_EXT_DEST_DIR"
+    if [[ -e "$PI_EXT_DEST" || -L "$PI_EXT_DEST" ]]; then
+        if link_if_into_repo "$PI_EXT_DEST"; then
+            ok "pi extension already linked"
+            return 0
+        fi
+        # Non-symlink (or foreign symlink) in the way — back it up rather
+        # than clobber. User may have local edits they want to preserve.
+        local backup="${PI_EXT_DEST}.bak.$(date +%Y%m%d-%H%M%S)"
+        mv "$PI_EXT_DEST" "$backup"
+        warn "Existing $PI_EXT_DEST backed up to $backup"
+    fi
+    ln -s "$PI_EXT_SRC" "$PI_EXT_DEST"
+    ok "Linked mempalace.ts → $PI_EXT_SRC"
+    printf '       Restart pi to load the extension (it reads ~/.pi/agent/extensions/\n'
+    printf '        at startup only).\n'
+}
+
 do_install() {
     echo
     echo "mempalace-toolkit installer"
@@ -227,6 +265,9 @@ do_install() {
     echo "==> Installation plan:"
     echo "   Symlink executables in bin/ into $BIN_DEST"
     echo "   Symlink SKILL.md into $SKILL_DEST"
+    if [[ -d "$PI_EXT_DEST_DIR" ]]; then
+        echo "   Symlink extensions/pi/mempalace.ts into $PI_EXT_DEST"
+    fi
     echo
     confirm || { echo "Aborted."; exit 0; }
     echo
@@ -234,6 +275,8 @@ do_install() {
     echo
     install_skill
     echo
+    install_pi_extension
+    echo
     check_path
     echo
     check_wake_up_protocol
@@ -278,6 +321,15 @@ do_uninstall() {
         ok "No skill symlink to remove"
     fi
 
+    echo
+    note "Removing pi extension symlink"
+    if link_if_into_repo "$PI_EXT_DEST"; then
+        rm "$PI_EXT_DEST"
+        ok "Removed pi extension symlink"
+    else
+        ok "No pi extension symlink to remove"
+    fi
+
     # Remove the marker and the now-empty skill directory, but only if
     # the marker was written by us and the directory has nothing else in it.
     local marker="$SKILL_DEST_DIR/.skill-source"