add: HEARTBEAT.md and sm-state.json

HEARTBEAT.md

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+# SM Heartbeat
+
+On every heartbeat:
+1. Read `memory/sm-state.json`
+2. Use the `plane` tool to `list_issues` 
+3. Check each issue against the state machine in AGENTS.md
+4. For stale items: check sm-state.json, ping if needed via `send_message`
+5. Update sm-state.json with any pings sent
+6. If first check after 9:00 and daily_summary_sent != today: post daily summary to #main
+
+If nothing needs attention: reply HEARTBEAT_OK

memory/sm-state.json

@@ -0,0 +1,5 @@
+{
+  "pings": {},
+  "last_scan": null,
+  "daily_summary_sent": null
+}