* stage bg_task trio combined (#2979 superset) on master for deep review
* fix(bg_task): unsubscribe SessionChannel on header-write failure (Codex deep-review catch) + regression test
* test: realign on-subscribe-recovery anchor to subscribe_to_session_channel after leak fix
* CHANGELOG: bg_task trio as v0.51.340 LD (HELD pending independent review)
* bg_task trio: apply 3 independent-review (greptile) fixes
1. start_session_turn now threads the session PROFILE model defaults
(_read_profile_model_config) into the wakeup model-resolve, so a brand-new
session with an empty model falls back to the profile default not global
DEFAULT_MODEL. Updated the white-box spy test signature accordingly.
2. /api/session/stream omits the Connection header (HTTP/1.1 keep-alive
default) to match the #3103 long-lived-SSE pattern.
3. Reaper now prunes _LAST_EMIT_TS for collected sessions so the coalesce
timestamp map can't grow one permanent entry per session forever.
nesquena APPROVED the PR; these are the 3 non-blocking greptile suggestions.
* test: realign _start_session_turn adapter stub lambda to new profile-defaults signature
