[BUG]: Status.get() returns idle for a session absent from the status map

Describe the bug

packages/opencode/src/session/status.ts — set(sessionID, status) publishes Event.Status on transition, and on type === "idle" publishes Event.Idle and deletes the map entry. get() then returns { type: "idle" } for any session with no entry, so "absent" and "idle" are indistinguishable to every consumer.

That makes idle a positive claim about a session that may not exist. The status map is InstanceState.make(… new Map()) — per-process and non-durable — so after a host restart every session reads idle, including sessions that were in-flight when the process died. Any consumer gating completion, cleanup, or notification on idle sees a fleet-wide false quiesce at exactly the moment the truth is unknown. The failure is silent and reads as health.

Worth documenting alongside: idle fires on all termination shapes — clean, error, and interrupt — so it is a quiesce barrier, not a success signal. The absent/idle conflation invites reading it as success.

Expected behavior

Absent and idle should be distinguishable — get() returning undefined (or an explicit unknown) for a missing entry, with idle reserved for an observed transition. Consumers can then treat unknown as "must check durably" rather than "finished".

The durable side already carries the discriminator: message.data.time.completed, set at packages/opencode/src/session/processor.ts:595, is persisted and survives restart — present on 49,961 of 50,000 sampled assistant messages on a live database. A session whose latest assistant message lacks it was in-flight when the process died. The information exists; the status surface collapses it.

For contrast within the same area, the field validation in packages/opencode/src/plugin/shared.ts throws precisely on every malformed input rather than returning a default that reads as success.

Steps to reproduce

1. Start a session and let it go idle, or start one and kill the process mid-turn.
2. Call the status surface for a session ID that has no map entry (any session after a restart, or any session that has already gone idle once).
3. Observe { type: "idle" } returned for a session the process has no state for.

Note: these steps are derived from the mechanism rather than from a recorded run — the finding came from reading the code path.

Environment

• Version: reproduces on current dev
• Platform: Linux (not platform-specific — the mechanism is in-memory state plus a defaulted read)

Additional context

Found while integrating an external supervisor process that consumes opencode session status to decide when a background task has finished. It surfaced there as a false-completion vector: the supervisor's candidate check reads idle, and on a host restart every session presents as idle at once.

A consumer can defend against this by treating "absent from the status map" as a distinct unknown state rather than as idle, but only if it already assumes the surface is lossy. A consumer taking get() at face value has no way to tell the two apart.