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/session.spec.ts

@@ -112,7 +112,7 @@ describe('Session', () => {
     session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
     session.append('turn/end', { turn: 1, reason: { kind: 'aborted' } })
     const replayed = new Session(SessionId('aborted-replay'), structuredClone(session.events))
-    expect(replayed.events).toEqual(session.events)
+    expect(replayed.events.slice(0, -1)).toEqual(session.events)
     const turnEnd = replayed.events.findLast(event => event.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toEqual({ kind: 'aborted' })
   })
@@ -188,7 +188,10 @@ describe('Session', () => {
 
     const replayed = new Session(SessionId('s3-replay'), [...original.events])
     expect(replayed.deriveMessages()).toEqual(original.deriveMessages())
-    expect(replayed.seq).toBe(original.seq)
+    // The seed verbatim, plus the boundary the constructor appends over it.
+    expect(replayed.events.slice(0, original.seq)).toEqual(original.events)
+    expect(replayed.seq).toBe(original.seq + 1)
+    expect(replayed.firstLiveSeq).toBe(original.seq)
   })
 
   it('rejects pre-provider request headers and assistant messages on seed/load', () => {
@@ -217,7 +220,7 @@ describe('Session', () => {
     const unrelatedPrimitiveData = {
       type: 'plugin/event', seq: 0, time: 1, data: null,
     } as unknown as SessionEvent
-    expect(new Session(SessionId('primitive-plugin-data'), [unrelatedPrimitiveData]).events)
+    expect(new Session(SessionId('primitive-plugin-data'), [unrelatedPrimitiveData]).events.slice(0, 1))
       .toEqual([unrelatedPrimitiveData])
   })
 
@@ -522,7 +525,8 @@ describe('Session', () => {
       { type: 'turn/end' as const, seq: 2, time: 3, data: { turn: 1, reason: { kind: 'completed' as const } } },
     ] as SessionEvent[]
     const session = new Session(SessionId('seed-ok'), goodSeed)
-    expect(session.events).toHaveLength(3)
+    expect(session.events.slice(0, 3)).toEqual(goodSeed)
+    expect(session.firstLiveSeq).toBe(3)
   })
 
   it('reads each seed array entry once so validation and storage use the same event', () => {
@@ -546,7 +550,7 @@ describe('Session', () => {
     const session = new Session(SessionId('seed-entry-snapshot'), seed)
 
     expect(reads).toBe(1)
-    expect(session.events).toEqual([accepted])
+    expect(session.events.slice(0, 1)).toEqual([accepted])
   })
 
   it('reads a nested seed-data getter once and stores its first JSON value', () => {
@@ -624,7 +628,7 @@ describe('Session', () => {
 
     const session = new Session(SessionId('seed-null-prototype'), [event])
 
-    expect(session.events).toEqual([{ ...event }])
+    expect(session.events.slice(0, 1)).toEqual([{ ...event }])
   })
 
   it('reads a nested seed-metadata getter once and stores its first JSON value', () => {
@@ -1647,6 +1651,7 @@ describe('todo/write event', () => {
     const replayed = new Session(SessionId('t4-replay'), [...original.events])
     expect(replayed.events.findLast(e => e.type === 'todo/write')!.data.todos)
       .toEqual([{ content: 'only', status: 'completed' }])
-    expect(replayed.seq).toBe(original.seq)
+    expect(replayed.events.slice(0, original.seq)).toEqual(original.events)
+    expect(replayed.firstLiveSeq).toBe(original.seq)
   })
 })