fix(runtime): discover subagent files created after watch attaches

[zawakin]

Problem

Subagents (e.g. Claude Code Task/Explore agents) sometimes never appear in agtrace watch, even though agtrace session show displays them correctly.

Root cause analysis

Analyzed against a real session where an Explore agent was launched mid-session. Claude Code writes the subagent transcript to {log_root}/{session_id}/subagents/agent-{id}.jsonl (with isSidechain: true and the parent sessionId in the header) after the Task tool call. All the data needed for parsing and spawn-linking is present in the logs.

The gap was in SessionStreamer (agtrace-runtime):

1. session_files was a fixed snapshot captured at attach time; handle_fs_event ignored any path not in that list. A subagent file created after watch attaches therefore never entered the stream, so the watch TUI never received its child-stream sessions.
2. Only EventKind::Modify was handled, so file creation events were dropped as well.

This explains the "sometimes" symptom: subagents that already existed before attach were picked up by the initial scan, while any subagent spawned during watching was invisible.

Fix

• StreamContext now owns the session file set and grows it dynamically: on a Create/Modify event for an unknown path, the file is adopted if discovery.probe() matches and its header session_id equals the watched session.
• Files confirmed to belong to another session are negatively cached to avoid re-reading headers on every poll tick; unreadable/empty files are retried on later events.
• EventKind::Create is now handled alongside Modify.

Testing

• New integration test streamer_subagent.rs reproduces the exact scenario (attach first, create subagents/agent-*.jsonl afterwards) and asserts a StreamId::Sidechain session reaches the stream. It fails on main (timeout) and passes with this fix.
• mise run verify passes.
• Manually verified against the real session logs: session show renders the sidechain stream with correct spawn context (spawned by Turn #2, Step #2), and console watch attaches and streams live.

Known limitation (out of scope)

WorkspaceSupervisor still intentionally skips sidechain files when emitting SessionUpdated (to avoid session switching), so sidechain-only activity does not bump a session's liveness for auto-attach. The attached-session streaming path — the reported symptom — is fully fixed here.

🤖 Generated with Claude Code