chore: add state-hub offline inbox and FR for ingest support

- state-hub-inbox/: convention for queuing progress events during
  degraded-mode sessions (no tunnel to State Hub)
- First pending event: Railiance01 bootstrap milestone (T03-T05)
- contrib/feature-requests/: FR for automated inbox ingest in state-hub
- README documents the drain procedure until automation is in place

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub-inbox/README.md

@@ -0,0 +1,35 @@
+# state-hub-inbox
+
+Offline event queue for State Hub sessions run without an active reverse tunnel.
+
+When the State Hub MCP server is unreachable, Claude writes pending events here
+as YAML files instead of dropping them. On the next connected session, these are
+drained by calling the appropriate State Hub write tools and marking each file
+`status: ingested`.
+
+## File format
+
+```yaml
+type: progress_event          # or: decision, task_status_update
+topic_id: <uuid>
+workstream_id: <uuid>
+event_type: milestone
+summary: "..."
+detail: {}
+status: pending               # → ingested after processing
+recorded_at: "YYYY-MM-DD"
+source_repo: railiance-hosts
+```
+
+## Drain procedure (manual until automated)
+
+At session start, if `get_domain_summary()` succeeds, check for pending files:
+
+```bash
+grep -l "status: pending" state-hub-inbox/*.yaml
+```
+
+For each, call `add_progress_event()` with the file's fields, then update
+`status: pending` → `status: ingested` and commit.
+
+See also: `contrib/feature-requests/fr-2026-03-08--threephoenix--state-hub--offline-inbox-ingest.md`