Implements bullet-proof configuration management to completely eliminate
allowed_nodes corruption by design. This builds on Phase 1 (file-only mode)
by replacing all shell/Python config manipulation with proper Go tooling.
**New Features:**
- `pulse-sensor-proxy config validate` - parse and validate config files
- `pulse-sensor-proxy config set-allowed-nodes` - atomic node list updates
- File locking via flock prevents concurrent write races
- Atomic writes (temp file + rename) ensure consistency
- systemd ExecStartPre validation prevents startup with bad config
**Architectural Changes:**
1. Installer now calls config CLI instead of embedded Python/shell scripts
2. All config mutations go through single authoritative writer
3. Deduplication and normalization handled in Go (reuses existing logic)
4. Sanitizer kept as noisy failsafe (warns if corruption still occurs)
**Implementation Details:**
- New cmd/pulse-sensor-proxy/config_cmd.go with cobra commands
- withLockedFile() wrapper ensures exclusive access
- atomicWriteFile() uses temp + rename pattern
- Installer update_allowed_nodes() simplified to CLI calls
- Both systemd service modes include ExecStartPre validation
**Why This Works:**
- Single code path for all writes (no shell/Python divergence)
- File locking serializes self-heal timer + manual installer runs
- Validation gate prevents proxy from starting with corrupt config
- CLI uses same YAML parser as the daemon (guaranteed compatibility)
**Phase 2 Benefits:**
- Corruption impossible by design (not just detected and fixed)
- No more Python dependency for config management
- Atomic operations prevent partial writes
- Clear error messages on validation failures
The defensive sanitizer remains active but now logs loudly if triggered,
allowing us to confirm Phase 2 eliminates corruption in production before
removing the safety net entirely.
This completes the fix for the recurring temperature monitoring outages.
Related to Phase 1 commit 53dec6010
