background subagents v2: session-scoped detach (durable outbox + child supervisor)

Context

The bg-subagents arc (2d0bb4f..0bdcc21) shipped run-scoped background subagents: a background child is cancelled+drained when its parent run ends. That bound is structural, not a preference — child events ride the run's event channel (closed at run end) and delivery is run-stream-coupled, so a child completing with no live run has no client, no outbox, no notification channel. See docs/design/BACKGROUND-SUBAGENTS.md (D8 + the v2 section).

This is the agents half follow-up to #23; the commands half (background Bash) remains there.

Shape (from the design doc)

• context.WithoutCancel detachment of the child goroutine from the run ctx tree.
• A Service-owned child supervisor (lifecycle beyond any single run).
• A durable per-session completion outbox so events/results survive with no live stream, drained on the next Converse/SSE attach.
• Re-attach semantics + server-shutdown draining.
• Everything shipped in v1 carries forward unchanged: the child-run registry, the session-id handle convention, SubagentStatus (poll + wait_ms), and the turn-boundary notice seam — only cancel-scope and event routing change.

Acceptance sketch

A background child started in run A delivers its completion notice at the start of run B on the same session; mecatui shows the detached child across runs; server shutdown drains or persists cleanly; the leak gates stay green.