feat: implement health checks in daemon event loop (bd-gqo)

Add health checks to checkDaemonHealth() function:
- Database integrity check using PRAGMA quick_check(1)
- Disk space check with 100MB warning threshold (platform-specific)
- Memory usage check with 500MB heap warning threshold

Platform-specific disk space implementations:
- Unix: uses unix.Statfs
- Windows: uses windows.GetDiskFreeSpaceEx
- WASM: returns unsupported (false)

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

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

cmd/bd/daemon_health_wasm.go

@@ -0,0 +1,10 @@
+//go:build wasm
+
+package main
+
+// checkDiskSpace returns the available disk space in MB for the given path.
+// Returns (availableMB, true) on success, (0, false) on failure.
+// WASM builds don't support disk space checks.
+func checkDiskSpace(path string) (uint64, bool) {
+	return 0, false
+}