WIP: Extract daemon runtime into internal/daemonrunner

- Created config.go with Config struct
- Created daemon.go with Daemon struct and Start/Stop methods
- Created logger.go for logging setup
- Created process.go for lock/PID management
- Created fingerprint.go for database validation
- Created flock_unix.go/flock_windows.go for platform-specific locking
- Created git.go for git operations

Still TODO:
- Implement runGlobalDaemon, startRPCServer, runSyncLoop
- Create sync.go, rpc.go, jsonl.go, validation.go
- Update cmd/bd/daemon.go to use daemonrunner

Part of bd-5f26

internal/daemonrunner/flock_windows.go

@@ -0,0 +1,35 @@
+//go:build windows
+
+package daemonrunner
+
+import (
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// flockExclusive acquires an exclusive non-blocking lock on the file using LockFileEx
+func flockExclusive(f *os.File) error {
+	// LOCKFILE_EXCLUSIVE_LOCK (2) | LOCKFILE_FAIL_IMMEDIATELY (1) = 3
+	const flags = windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY
+
+	// Create overlapped structure for the entire file
+	ol := &windows.Overlapped{}
+
+	// Lock entire file (0xFFFFFFFF, 0xFFFFFFFF = maximum range)
+	err := windows.LockFileEx(
+		windows.Handle(f.Fd()),
+		flags,
+		0,          // reserved
+		0xFFFFFFFF, // number of bytes to lock (low)
+		0xFFFFFFFF, // number of bytes to lock (high)
+		ol,
+	)
+
+	if err == windows.ERROR_LOCK_VIOLATION || err == syscall.EWOULDBLOCK {
+		return ErrDaemonLocked
+	}
+
+	return err
+}