docs: Add watchdog chain documentation for Boot/Deacon lifecycle (gt-1847v)

Creates docs/watchdog-chain.md explaining the Daemon/Boot/Deacon architecture:
- Why two agents (Boot is ephemeral triage, Deacon is persistent patrol)
- Session ownership (gt-deacon-boot vs gt-deacon)
- Heartbeat mechanics and freshness thresholds
- Boot decision matrix (start/wake/nudge/nothing)
- Design decision: keep separation, fix implementation bugs

Cross-references added to operational-state.md and understanding-gas-town.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/operational-state.md

@@ -102,6 +102,9 @@ bd list --type=role --label=mode:degraded
 
 ## Boot: The Deacon's Watchdog
 
+> See [Watchdog Chain](watchdog-chain.md) for the complete Daemon/Boot/Deacon
+> architecture and design rationale.
+
 Boot is a dog (Deacon helper) that triages the Deacon's health. The daemon pokes
 Boot instead of the Deacon directly, centralizing the "when to wake" decision in
 an agent that can reason about it.