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-query/session-query-sqlite/tests/sqlite.spec.ts

@@ -1077,8 +1077,9 @@ describe('SQLite reconciliation and source lifecycle', () => {
     await expect(ctx.sessionQuery.searchEvents({ sessionId: live.id, query: 'needle' }))
       .rejects.toThrow(expectCode('SESSION_QUERY_INDEX_FAILED'))
     db.exec('PRAGMA query_only = OFF')
+    // seq 2: one-event seed, its boundary, then the message appended above.
     await expect(ctx.sessionQuery.searchEvents({ sessionId: live.id, query: 'needle' }))
-      .resolves.toMatchObject({ items: [{ seq: 1 }] })
+      .resolves.toMatchObject({ items: [{ seq: 2 }] })
   })
 })