fix(coding-agent): honest status and prompt eviction for empty sessions

[snimu]

Two empty-session lifecycle fixes, found while investigating the macOS "ctrl+c never ends the session / after a couple of sessions it can't create a new one" report (companion to #1918).

Status honesty

A 0-message detached session was pinned at "working" forever - rendered as a red running entry in the agents view and prime-agent list - because activeActivityForSession holds sessions at "working" until a summarizer verdict exists, and an empty session never produces one. Users reasonably concluded that exiting "never ends the session". The fix is the same carve-out finished subagents already have: a session with 0 messages and nothing in flight is idle. An accepted in-flight first prompt still shows "working" (the busy guard runs first, pinned by test).

Evict empty unnamed sessions on last-client disconnect

An abandoned empty session parked a resident worker (~80 MB) plus an IPython kernel for the 90-minute idle-eviction window, holding 2 daemon file descriptors - the accumulation ingredient behind the EMFILE failure fixed in #1918. Now, when the last client disconnects from a worker whose sessions are all empty (0 messages), unnamed, not busy, and hold no heartbeat/cron registration, the supervisor passivates it immediately through the existing eviction path. Action-driven only - no new timers. Naming a session exempts it (that is the "I want to come back" signal); client-owned workers keep their existing cleanup path; eviction remains passivation, so the on-disk session survives as a draft.

Races verified against the ownership machinery from #1756/#1864: a create/attach arriving mid-stop fails cleanly with "worker is stopping" and relaunches from the draft; the eviction re-checks worker identity after its summary refresh and cannot stop a relaunched successor (stopWorker additionally pins the entry pid). Cold reopen of an evicted draft measures ~0.55s on macOS, indistinguishable from a warm attach.

Deliberately not gated on idleEvictionMinutes: "off": everything a user wants kept warm (messages, a name, a schedule, an attached client) already blocks this eviction, and gating would reinstate the leak for exactly the users most exposed to it.

Tests

Six behavior tests plus one extended assertion across daemon-session-list and a new daemon-supervisor-eviction case set: empty-idle status (proven red), the in-flight-first-prompt guard, last-client-only eviction (proven red), named/busy/non-empty exemptions, schedule-pinned and client-owned exemptions, and a replaced-worker race guard.

Linear: ENG-5809

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Note

Medium Risk
evictEmptySessionOnLastDetach stops workers on detach; reconnecting spins up a new worker, and fence/drain logic must stay correct alongside idle sweeps and mid-flight mutations.

Overview
Fixes two empty-session lifecycle issues in the coding-agent daemon: misleading "working" status and workers staying resident after abandon.

Status: activeActivityForSession now treats zero-message resident sessions as idle (like finished subagents), so prime-agent list and the agents view no longer show abandoned empty drafts as permanently running. Sessions with real in-flight work still report working first.

Eviction: When the last client detaches or disconnects, the supervisor may passivate the worker immediately if every session on it matches isEvictableEmptySessionSummary (no messages, no name, not busy, no heartbeat/cron). Named sessions, busy sessions, client-owned workers, and multi-attach cases are exempt; eviction reuses the idle-sweep mutation drain + fence (withEvictionFence) so detach eviction and periodic idle sweeps do not race. On-disk draft sessions are preserved.

Tests cover empty-idle classification, detach-only eviction, exemptions, worker-replacement races, and fence coordination with idle sweeps.

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Note

Fix coding-agent daemon to report idle status and evict empty sessions on detach

• Empty resident sessions with zero messages and nothing in flight now report activity as "idle" instead of staying pinned at "working" in daemon-session-list.ts.
• When the last client detaches from an empty, unnamed, idle session, the worker is immediately stopped and its on-disk draft persists, via daemon-supervisor.evictEmptySessionOnLastDetach.
• Eviction runs under a reusable eviction fence (withEvictionFence) that drains pending mutations before the decision, preventing races with in-flight worker refreshes or schedules registered during the drain.
• Behavioral Change: daemon-supervisor client disconnect and detach command handlers now trigger eviction for qualifying empty sessions; sessions that were previously kept alive will be passivated immediately.

^{Macroscope summarized 2114326.}

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