Summary

• Add hud_state.py module managing ~/.codingbuddy/hud-state.json for cross-hook state sharing
• read_hud_state() with shared lock (LOCK_SH), returns {} on any error
• init_hud_state() creates initial session state (timestamp, version, mode, agent)
• update_hud_state() atomic read-modify-write under single exclusive lock (LOCK_EX)
• Follows stats.py fcntl file locking patterns
• 9 tests across 4 test classes, all passing
• 356 existing tests unaffected (zero regression)

Test plan

• python3 -m pytest tests/test_hud_state.py -v — 9 passed
• python3 -m pytest tests/ -q — 356 passed
• lint, format:check, typecheck, test:coverage, circular, build — all passed
• Security audit (all workspaces) — no issues

Closes #1087