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_unix.go

@@ -0,0 +1,18 @@
+//go:build unix
+
+package daemonrunner
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// flockExclusive acquires an exclusive non-blocking lock on the file
+func flockExclusive(f *os.File) error {
+	err := unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if err == unix.EWOULDBLOCK {
+		return ErrDaemonLocked
+	}
+	return err
+}