M168 finish — Phase 5 telemetry · Phase 6 docs · subagent signal

One bundle: #1915 (coordination telemetry) + #1916 (architecture docs + monitors purge) + #1874 (subagent BUDGET_EXHAUSTED). Closes umbrella #1908.

Coordination state lives at ~/.local/state/orchestkit/ — outside .claude/, machine-global across repos/worktrees. No background process; hooks read/write on events they already fire.