Implement exclusive lock protocol for daemon/external tool coexistence
- Add ExclusiveLock struct with JSON marshaling and validation
- Implement IsProcessAlive() with EPERM fail-safe behavior
- Add ShouldSkipDatabase() with stale lock cleanup
- Integrate lock checking into daemon sync cycle
- Return holder name on stale removal for better logging
- Case-insensitive hostname comparison
- Comprehensive unit tests (89.3% coverage)
- Documentation updates (ADVANCED.md, AGENTS.md)
- Add .beads/.exclusive-lock to .gitignore
Closes bd-115, bd-116, bd-117, bd-118, bd-119, bd-120, bd-121, bd-122
Amp-Thread-ID: https://ampcode.com/threads/T-0b835739-0d79-4ef9-aa62-8446a368c42d
Co-authored-by: Amp <amp@ampcode.com>
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