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/src/components/dialogs.ts

@@ -23,6 +23,7 @@ import {
   type AgentLlmTarget,
 } from '@deepseek-ai/dsh-agent'
 import type { LlmModelInfo, LlmModelReasoningInfo, ReasoningEffortId } from '@deepseek-ai/dsh-llm'
+import { lastActivityTime } from '@deepseek-ai/dsh-session'
 import type { SessionId } from '@deepseek-ai/dsh-session'
 import { foldGoal, type GoalPhase } from '@deepseek-ai/dsh-goal'
 import { foldSessionTitle } from '@deepseek-ai/dsh-session-title'
@@ -512,7 +513,8 @@ export function summarizeResumeCandidate(
   return {
     record,
     title,
-    lastActivityAt: snapshot.events.at(-1)?.time ?? snapshot.session.createdAt,
+    // Excludes a prior pickup's boundary, or every browsed session floats up.
+    lastActivityAt: lastActivityTime(snapshot.events) ?? snapshot.session.createdAt,
     lastTurn: resumeTurnLabel(snapshot),
     currentWorkspace: record.header.cwd === cwd,
     workspaceLabel: formatWorkspace(record.header.cwd),