(activity): light has-busy-agents for remote parents and local parents without a PTY

[devsuitup]

Symptom

.has-busy-agents (the parent row marker) only lights for a parent that is in activeSessions: activeSubagentsByParent is fed by the subagent-spawned / subagent-completed IPC, emitted by detectSubagentTransitions() (session-transitions.js), which scans activeSessions only.

Two parents never light:

• a remote parent, even though the watch channel ((remote): push liveness over a persistent watch channel #241) is recursive and hears every agent-<id>.jsonl write — sessionIdFromRel in remote-activity.js requires a UUID basename, so subagent events are dropped rather than attributed;
• a local parent launched outside Switchboard.

Read, not executed: confirm with a live subagent on the host before coding.

Fix

In the source adapter (see the domain-layer issue), attribute a subagent transcript event to its parent by walking up from the path — enumerateSessionFiles() in read-session-file.js knows the layouts — and emit subagentSpawned / subagentCompleted (or a decaying agentsBusy) for the parent, whatever the source.

Acceptance

• A remote parent whose subagent is writing shows has-busy-agents within the watch coalescing window.
• Same for a local parent with no PTY of ours.
• The attribution function is pure and tested against the real subagent layouts; a mutation dropping the parent resolution makes it red.

[devsuitup]

Step 4 of the alignment sequence: #244 → #245 → #246 → #247. Depends on the adapter ports of #246.