Feature: session lifecycle events (session-opened / session-closed) on the event bus

[jumski]

Context

The pi extension already exposes an event bus contract third-party extensions build on:

• plannotator:request — external callers can start annotate / annotate-last / code-review / plan-review sessions and await the decision
• plannotator:review-result — emitted when a channel-initiated plan review completes
• plannotator:plan-approved — emitted on plan approval

Gap

Nothing is emitted when a browser session opens or closes. The built-in slash commands (/plannotator-annotate, /plannotator-last, /plannotator-review, /plannotator-plan-mode) open sessions directly without touching the request channel, so from outside the extension there is no way to know "a review is open and waiting on the human".

Use case

Agent harnesses and terminal multiplexers can surface waiting state to the user. Example: while an agent has a plannotator browser session open, we show a "🗨 needs review" badge on the agent's pane in Herdr (set via pane metadata while the session is open, cleared when it closes). Today this is only possible for sessions we start ourselves through plannotator:request; sessions opened by the built-in slash commands are invisible to us.

Proposal

Emit two channels, mirroring the existing pattern:

• plannotator:session-opened — { action: "annotate" | "annotate-last" | "code-review" | "plan-review", reviewId?: string, url: string }
• plannotator:session-closed — same identity fields plus { outcome: "closed" | "submitted" | "approved" }

If both are emitted from the single place sessions start and finish (rather than per entry point), every caller — slash commands, channel requests, stop-hook flows — gets covered automatically.

Version observed: @plannotator/pi-extension 0.27.9.

[backnotprop]

Thanks @jumski, well specified. This is the same underlying need as #1582 (tell the user when a session is waiting), filed this weekend, and the Herdr use case there is the same one. Keeping this open as the concrete Pi event-bus shape (session-opened / session-closed mirroring the existing channels); the owner will rule on it. No release timing yet.