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/session-persistence/session-persistence-jsonl/tests/jsonl.spec.ts

@@ -378,7 +378,9 @@ describe('SessionPersistenceJsonl: durability and crash semantics', () => {
     await ctx.sessions.flush(child)
     const loaded = await ctx.sessionPersistence.load(child.id)
 
-    expect(loaded.events).toEqual(source.events)
+    // The inherited prefix reaches disk verbatim, then the child's boundary.
+    expect(loaded.events.slice(0, source.events.length)).toEqual(source.events)
+    expect(loaded.events.at(-1)).toMatchObject({ type: 'session/inherited', seq: source.events.length })
     expect(loaded.meta).toMatchObject({
       id: SessionId('persist-child'),
       cwd: '/workspace',