feat(session): project the inherited-history boundary into the log

A plugin owning a standalone open/close bracket cannot tell a dead marker
from a live one: an unmatched `compact/start` reads identically whether the
previous writer died mid-compaction or a compaction is running now.
`Session.firstLiveSeq` already holds that answer exactly, but only in memory.

Append the log-only `session/inherited` event at that seq from the seeded
constructor — the single waist all six seeded-start paths pass through
(resume, configured startup on a persisted id, `sessions.fork()`, a subagent
fork child, `adopt()`'s live prefix, and a bare seeded `create`). Read it
through the new `isInheritedSeq(events, seq)`.

The constructor placement means persistence needs no changes: the marker is
already in `events` when a backend captures the creation seed, so it rides
the ordinary seed path with no load-time write. It also covers fork, where
the inherited bracket's owner may still be running — the case a
persistence-layer boundary could not reach.

Activity ordering excludes the boundary through `lastActivityTime()`, since
lazy resume makes browsing a pickup and the three call sites would otherwise
float every opened session to the top of a picker or list.

packages/core/agent-loop/tests/loop.spec.ts

@@ -1207,8 +1207,9 @@ describe('agent loop', () => {
 
     const replayed = ctx.sessions.create(SessionId('replayed'), { seed: [...agent.session.events] })
     expect(replayed.deriveMessages()).toEqual(agent.session.deriveMessages())
-    // event-by-event identity of types
-    expect(replayed.events.map(e => e.type)).toEqual(
+    // event-by-event identity of types over the inherited prefix
+    expect(replayed.events.slice(0, agent.session.seq).map(e => e.type)).toEqual(
       agent.session.events.map(e => e.type))
+    expect(replayed.events.at(-1)?.type).toBe('session/inherited')
   })
 })