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/session/tests/properties.spec.ts

@@ -112,7 +112,19 @@ describe('Session properties', () => {
       const original = build(events)
       const replayed = new Session(SessionId(`replay-${counter++}`), [...original.events])
       expect(replayed.deriveMessages()).toEqual(original.deriveMessages())
-      expect(replayed.seq).toBe(original.seq)
+      // A non-empty replay grows by exactly one log-only boundary.
+      expect(replayed.events.slice(0, original.seq)).toEqual(original.events)
+      expect(replayed.seq).toBe(original.seq === 0 ? 0 : original.seq + 1)
+    }))
+  })
+
+  it('replaying an already-inherited log adds no further boundary', () => {
+    fc.assert(fc.property(logArb, (events) => {
+      const original = build(events)
+      const once = new Session(SessionId(`idem-a-${counter++}`), [...original.events])
+      const twice = new Session(SessionId(`idem-b-${counter++}`), [...once.events])
+      // Lazy resume makes browsing a pickup, so this must not grow per open.
+      expect(twice.events).toEqual(once.events)
     }))
   })