fix: resume active runs for job notifications

[sam-saffron-jarvis]

What

Finishes the queue_agent notify_when_done web behavior for both active and idle loaded sessions.

Two cases were broken/underspecified:

1. Active web session: if a queued job finished while the origin session was streaming final prose, the notification interjection could be queued but left pending because the engine reached a text-only terminal turn and emitted done.
2. Idle loaded web session: if the origin response had already ended, there was no stream to interject into; the old path only appended a transcript note, so nothing continued.

This PR now handles both:

• active session → queue an internal AutoContinue notification interjection and continue at text-only turn boundaries
• idle but loaded web session → start a detached response run with the job notification as the next user message, so the assistant actually reacts
• unloaded/missing runtime → keep the safe store-only assistant notification fallback
• normal user interjections remain pending/cancellable
• FIFO is preserved: normal user interjections block later auto-continue job notices from jumping the queue

Why

notify_when_done should mean “tell the originating conversation and let it carry on,” not “leave a note somewhere and hope the human notices.” Waiting is the bad path; notification should be the good path.

Review notes

Reviewed with claude-bin:opus-max. I folded in the correctness nits before opening/pushing:

• limited auto-continue to the pure text-only path, avoiding unregistered/passthrough client tool-call abandonment
• removed the precheck/drain TOCTOU by draining once and branching on the result
• avoided converting a last-turn completion into MaxTurnsExceededError; if no turn budget remains, the notice stays pending and the current response completes
• added FIFO-blocking coverage
• added loaded-idle-session coverage proving a continuation response run is started and persists user notice + assistant response

Verification

go test ./internal/llm -run 'Interjection_(NoToolCalls|AutoContinue)'
go test ./cmd -run 'JobsV2Notify|Interrupt|Interjection'
go build ./...
go test ./...