fix: re-register forked/replaced sessions with the broker

[iRonin]

Summary

Forked/replaced sessions never re-register with the broker. The runtime is initialized only in session_start; when a session's id changes in-process without a fresh session_start (forking/branching a session, or a resume path that adopts an id), it never (re-)registers and stays unreachable over intercom until a full process restart. No error surfaces — the session is alive and running turns locally but invisible to peers.

Fixes #44.

Root cause

• getLiveContext() returns null whenever ctx.sessionManager.getSessionId() !== currentSessionId.
• Every lifecycle handler is gated by getLiveContext() and bails first.
• currentSessionId is only advanced inside those gated handlers (e.g. turn_start).

So once the live id diverges from currentSessionId, the guard rejects every handler, currentSessionId can never catch up, and the broker registration is stranded under the old identity. A related case: if no session_start ever fires, the extension stays inert even while the session runs turns.

Fix

• Extract the session_start initialization into startSessionRuntime(ctx) (also drops any client still registered under the previous identity).
• Add adoptSessionContext(ctx), called from turn_start, which re-initializes when the runtime never started or the live session id diverged from the registered one. A turn_start only fires for a live session, so the disposed/shuttingDown flags (true both before the first session_start and after a real shutdown) are intentionally not used to gate adoption — startSessionRuntime resets them.

Minimal and protocol-neutral; no broker changes.

Tests

Adds two regression tests to intercom.integration.test.ts:

• a forked session that starts on its first turn (no session_start) registers with the broker — fails before the fix (session never appears); passes after.
• an in-process session fork re-registers under the new identity — with no /name, the presence name is derived from the session id; before the fix the broker keeps the stale pre-fork name, after the fix it switches to the new identity and drops the old one.

Verified fail-then-pass against the unpatched tree. Full suite green:

ℹ tests 38
ℹ pass 38
ℹ fail 0