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/subagent/subagent-inprocess/tests/inheritance.spec.ts

@@ -126,9 +126,10 @@ describe('in-process policy inheritance', () => {
 
       expect(child.session.header.seedLength).toBe(1)
       expect(child.session.firstLiveSeq).toBe(seed.length)
+      // seq 1 is the inherited-history boundary.
       expect(child.session.events.filter(event => event.type === 'sandbox/mode')).toMatchObject([
         { seq: 0, data: { mode: 'workspace-write' } },
-        { seq: 1, data: { mode: 'read-only', source: 'delegation' } },
+        { seq: 2, data: { mode: 'read-only', source: 'delegation' } },
       ])
       await expect(readFile(blocked, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' })
       expect(ctx.sandboxPolicy.overrideOf(child.session)).toBe('read-only')