docs(session): make the boundary's position and ownership conditional

Documentation only; no behavior change.

`firstLiveSeq`'s JSDoc still stated the boundary sits at that seq
unconditionally. Verified reachable on a plain production path: pick up a log,
do no work, pick it up again — the seed already ends in a boundary, so it is
not re-marked and `events[firstLiveSeq]` is undefined. Both that claim and the
firehose-gap sentence are now conditional, with the locate rule ("scan for the
last boundary") matching what types.ts already said.

`repair.ts`'s header claimed this module supplies the boundary. It does not —
the constructor does; this module supplies the activity read that must skip it.
Reworded, and it now names the closer timestamp leak, which is the one real
coupling that justifies `lastActivityTime` living beside the repair synthesis.

Recorded that `Session`'s constructor is the boundary's only legitimate writer,
since the invariant companion constrains nothing and a plugin-side append would
silently turn live brackets below it into dead history.

packages/core/session/src/repair.ts

@@ -1,8 +1,10 @@
 /**
  * Crash-recovery repair for an interrupted session log. It preserves a fully
  * written final turn and supplies the missing tool, step, and turn boundaries
- * needed to resume with a provider-valid transcript, plus the inherited-history
- * boundary a plugin-owned bracket reads to tell dead history from live work.
+ * needed to resume with a provider-valid transcript, plus the activity-time
+ * read that must skip the inherited-history boundary — which this module does
+ * not write (`Session`'s constructor does) but whose synthetic closers can
+ * inherit that boundary's timestamp, the one real coupling between the two.
  * @module @deepseek-ai/dsh-session/repair
  */