docs(FN-4404): document blocked heartbeat investigation preflight

- Add FN-4404 investigation notes under docs/
- Record the missing agent and heartbeat evidence gathered during preflight
- Document why the absent runtime state blocks Case A/B/C classification
- List the runtime data needed to unblock the remaining investigation steps

Fusion-Task-Id: FN-4404

docs/fn-4404-heartbeat-investigation.md

@@ -0,0 +1,25 @@
+# FN-4404 heartbeat investigation (blocked)
+
+## Status
+
+Blocked during preflight due missing source data.
+
+## Evidence collected
+
+- `.fusion/fusion.db` exists and is readable.
+- `sqlite3 .fusion/fusion.db "SELECT COUNT(*) FROM agents;"` returned `0`.
+- Querying for `name = 'Technical Writer'` returned no rows.
+- `fn_list_agents({ includeEphemeral: true })` returned no agents.
+
+## Why this blocks FN-4404
+
+The task requires measuring `Technical Writer` runtime config, `lastHeartbeatAt`, and heartbeat-run history, then classifying against engine/dashboard staleness thresholds. With no agent rows and no heartbeat runs present, there is no measurable state to classify and no evidence for Case A/B/C.
+
+## Unblock requirements
+
+Provide a project/runtime state (or DB snapshot) that contains:
+
+1. A `Technical Writer` agent record in the agent store.
+2. Corresponding heartbeat-run records for that agent.
+
+Once available, the investigation can be completed per PROMPT.md Steps 1–5.