[vcing]

Summary

• add a small, versioned registry that lets server-side extensions report exact-session liveness
• consult extension liveness only when the normal 10-minute idle timer would otherwise evict a session
• preserve explicit shutdown, reload/replacement, process exit, and Stop fallback cleanup semantics

Problem

Pi Web currently decides whether a session is idle from foreground AgentSession state only: queued prompts, streaming, compaction, and bash execution. An extension can therefore own detached work after the parent turn has ended while the host sees the session as idle.

At the timeout boundary, Pi Web disposes the session and its extension runtime. Detached work may continue elsewhere, but the extension state or completion notifier that was supposed to wake the parent is gone. This affects long-running background tasks and is the structural limitation described in #629 and #572.

A configurable global timeout such as #665 is useful operationally, but it cannot distinguish a disposable session from one with live extension-owned work. This PR is complementary: extensions opt in per exact session, and the ordinary timeout remains unchanged for every other session.

Design

The host publishes Symbol.for("@agegr/pi-web/session-liveness/v1") with a synchronous provider registry. A provider supplies:

• a name
• the exact sessionId and optional sessionFile
• a cheap synchronous isActive() probe

Each registration receives an independent idempotent release function. Providers are matched only against the wrapper's exact session identity. A matching provider that throws or returns a non-boolean value fails safe by preserving that session and logging the error.

AgentSessionWrapper.isRunning() is deliberately unchanged because it is also used for UI state and prompt/session coordination. The registry is consulted only inside the automatic idle-eviction callback, after forceShutdownOnIdle has been checked.

Validation

• env -u PI_WEB_PASSWORD npm test — 850 passed
• npm run lint
• npx tsc --noEmit
• focused session-liveness and shutdown tests — 24 passed
• git diff --check

Per the repository development guidance, next build was not run.

Related: #629, #572, #665

Companion provider-side integration: nicobailon/pi-subagents#1857 (merged as 11ea7e9). Until this host-side registry ships, that integration intentionally degrades to a no-op.