Background tasks show as running in UI but disappear from /tasks after a non-graceful exit

Bug: background tasks stuck as "running" in UI after the app exits, while /tasks can no longer find them

Repro

1. Start a long-running background task in a session (e.g. a shell command that sleeps).
2. Kill the app process abruptly (or let the machine reboot / the process die without graceful shutdown) — any exit path that does not go through the graceful session-close flow.
3. Reopen the app and look at the session in the web/desktop UI: the task still shows as running.
4. In the CLI, /tasks (or the REST endpoint GET /api/v1/sessions/{id}/tasks) reports nothing for that session.

Root cause

Task state lives in several places with no reconciliation between them:

• The live registry (AgentTaskService) is scoped to the agent/session and disappears when the session is destroyed — so the CLI/cold REST endpoint legitimately see no tasks.
• The wire log (wire.jsonl) records task.started as a durable event, but task.terminated is only written on the graceful-exit path (removeManaged -> stopAllOnExit -> flush). On a crash/kill the termination event is missing.
• The UI reads tasks from the transcript projection. For a cold session this is rebuilt purely from wire.jsonl, so a task with a bare task.started and no task.terminated is reconstructed forever as state: 'running'.

Result: two views of the same task that permanently disagree — "running" in the UI, empty in /tasks.

Suggested fix

Two small, storage-layer-only changes:

1. kap-server cold transcript read (readColdSnapshot): a cold session's owner process is gone by definition, so any non-terminal task in the folded snapshot should be rewritten to a terminal state ('lost').
2. AgentTaskService.dispose: settle non-terminal entries to killed (reason: session closed) and persist, mirroring the graceful-exit path, so a hard kill does not leave bare task.started events in the wire log.

Both changes are scoped to the server side; the CLI and live-session paths need no changes.

Start with the kap-server readColdSnapshot path and AgentTaskService.dispose, then trace the existing removeManaged → stopAllOnExit → flush flow and wire.jsonl task events. Verify that cold snapshots rewrite non-terminal tasks to lost, while disposal persists non-terminal entries as killed with the session-closed reason, and confirm the UI and /tasks views agree after an abrupt exit.