fix(coding-agent): report session id alongside path to herdr

[witt3rd]

Summary

The builtin herdr reporter captured the resumable session ref correctly but sent only agent_session_path when the session was persisted. Herdr's report handler accepts paths only for the pi/omp labels; for the prime-agent label it reads agent_session_id, so the ref was silently dropped and panes never became resumable (agent_session stayed null, no prime-agent --resume on herdr-server restart).

The reporter now sends both agent_session_id and agent_session_path on every report, so the session registers on current herdr (via id) and on path-accepting herdr (via path).

Fixes #1260.

Root cause notes

• The ref itself resolves fine: daemon-created sessions are persisted via SessionManager.create, which assigns the session file at construction, before session_start fires. A socket tap on HERDR_SOCKET_PATH shows the first pane.report_agent already carrying agent_session_path. The drop happens herdr-side, in session_ref_from_report.
• prime-agent support in herdr currently lives on a fork (witt3rd/herdr); upstream herdr has no ("herdr:pi","prime-agent") official source. The companion herdr-side fix (accept prime-agent paths, matching the snapshot/planner handling) is witt3rd/herdr@d524e4f1. This change stands alone: sending the id makes registration work regardless.

Test plan

• New regression test packages/coding-agent/test/suite/regressions/1260-herdr-session-ref.test.ts drives the builtin extension against a real persisted SessionManager and a fake herdr socket, asserting both identity forms on the report (fails without the fix).
• Extended test/herdr-agent-state.test.ts assertions for both forms.
• npm run check clean.
• Verified end-to-end on an isolated herdr session: agent start → prompt → agent_session registered → herdr-server restart → pane relaunches prime-agent --resume <ref> with session content intact.

Note

Fix herdr reporter to include session id alongside session path in reports

Previously, withSessionRef in herdr-agent-state.ts sent either agent_session_path or agent_session_id, but not both. Now both fields are included in pane.report_agent requests when available, allowing herdr to register a resumable session correctly.

^{Macroscope summarized dafafe8.}