fix(session-inbox-monitor): scope pi extension to own project inbox only

Per vitya's rule an agent may only read its own directory's inbox. The
first global-scan version (all ~/projects inboxes + PI_INBOX_ROOTS) was
scrapped same day; rewrote as session-scoped: watches only
<ctx.cwd>/.claude-inbox/, decoy test proves other inboxes untouched.

skills/session-inbox-monitor/SKILL.md

@@ -12,8 +12,8 @@ description: >
   «настрой авто-монитор инбокса», «raise inbox monitor», «auto-arm inbox
   watcher». Headless (`claude -p`): does NOT raise — Monitor doesn't work
   there; rely on the Stop-hook inbox pickup + Notify/ntfy. Pi (pi-coding-agent)
-  sessions: a global extension delivers any `.claude-inbox/` message to the
-  current session regardless of cwd (see the Pi section). NOT for how to
+  sessions: a global extension delivers the session's own project inbox
+  messages (own directory only, see the Pi section). NOT for how to
   handle a received message (→ inter-session-peer-discipline) nor the
   multi-machine inbox backend (→ cross-machine-inbox design).
 ---
@@ -129,26 +129,26 @@ for free on session end. Headless sessions skip it and rely on the pull-model
 - **No repo writes.** The hook and its `~/.claude/settings.json` registration are
   machine-local; only this skill (docs) and `.claude-inbox/` activity are in play.
 
-## Pi (pi-coding-agent) support — global extension
+## Pi (pi-coding-agent) support — session-scoped global extension
 
-Same contract, pi-native, and **not cwd-scoped**: which folder the pi session
-sits in is irrelevant — every `.claude-inbox/` under the configured roots is
-watched and swept; a message pages the *current* pi session with the project
-name in the header (`[inbox] <project>/<name>` + body inline, then moved to
-`.read/`).
+Same contract, pi-native, and **session-scoped**: the extension watches ONLY the
+current session's project inbox (`<ctx.cwd>/.claude-inbox/`) — it never reads
+other projects' inboxes (vitya's rule: an agent may only read its own
+directory's inbox). Installed globally so *every* pi session has the
+capability, but each session only ever touches its own project's inbox.
 
 - **Source of truth:** `.common/lib/pi-extensions/inbox-monitor.ts` (Node
   built-ins only, no npm deps). Deploy: copy to
   `~/.pi/agent/extensions/inbox-monitor.ts` (global → every pi, every
   directory), hot-reload with `/reload`. Test:
-  `node --experimental-strip-types .common/lib/pi-extensions/inbox-monitor.test.mjs`.
-- **Roots:** direct children of `~/projects` by default (all inboxes live there,
-  depth 1); override with `PI_INBOX_ROOTS` (pathsep-separated, `;` on Windows).
-- **PUSH:** one 15s global poll while a session is live (interactive only).
-  **PULL:** `agent_settled` sweep — the pi equivalent of the CC Stop-hook
-  pickup. Both share one dedup set per process; the `.read/` move is the
-  cross-process guard — first sweeper (CC hook or pi) claims the message, the
-  other skips it.
+  `node --experimental-strip-types .common/lib/pi-extensions/inbox-monitor.test.mjs`
+  (incl. decoy check — another project's inbox is never touched).
+- **Opt-in per project** — same as CC: `.claude-inbox/` dir exists OR CLAUDE.md
+  line `inbox monitor: raise on start`.
+- **PUSH:** 15s poll of the session's own inbox (interactive only). **PULL:**
+  `agent_settled` sweep — the pi equivalent of the CC Stop-hook pickup. Both
+  share one dedup set per process; the `.read/` move is the cross-process
+  guard — first sweeper (CC hook or pi) claims the message, the other skips it.
 - **Headless (`pi -p`, `ctx.hasUI === false`):** NO delivery — no watcher, no
   sweep. Messages sit in the inbox until an interactive session picks them up.
   Mirrors CC headless (external Notify there, nothing in-run) and avoids