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/ui/tui/tests/tui.snapshot.ts

@@ -837,6 +837,9 @@ describe('TUI terminal-state snapshots', () => {
         { type: 'step/end', seq: 5, time: Date.parse(`${day}T00:00:06Z`), data: { turn: 1, step: 1 } },
         { type: 'turn/end', seq: 6, time: Date.parse(`${day}T00:00:07Z`), data: { turn: 1, reason: { kind: 'completed' } } },
         { type: 'session/title', seq: 7, time: Date.parse(`${day}T00:00:08Z`), data: { title, messageSeqs: [1], source: { kind: 'fallback' } } },
+        // A prior pickup, dated well after the work: the picker must still
+        // show the work's date, not the pickup's.
+        { type: 'session/inherited', seq: 8, time: Date.parse('2026-07-23T07:59:00.000Z'), data: {} },
       ],
     })
     const harness = await setupSnapshot({