Add project-heartbeat skill skeleton

skills/project-heartbeat/references/state-machine.md

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+# Project heartbeat state machine
+
+## States
+- idle
+- armed
+- checking
+- closed
+
+## Normal flow
+1. main sends a project-progress update -> `armed`
+2. 10-minute silence timer runs
+3. meaningful update arrives before timeout -> reset timer, remain `armed`
+4. timeout fires -> `checking`
+5. main inspects workers/project state and sends user-visible update
+6. if still active -> back to `armed`
+7. if done/paused/failed/cancelled -> `closed`
+
+## Failure patterns
+- no worker trace after supposed dispatch -> `blocked (launch failure)`
+- ETA missed with no milestone -> `blocked (stalled)`
+- model/auth/tool/path issues -> `blocked` with explicit reason