feat(vmm): local VMM instance discovery

[kvinwang]

Summary

• Each dstack-vmm process registers itself in /run/dstack-vmm/<uuid>.json on startup with instance metadata (listen address, pid, working dir, config path, image/run paths, node name, version)
• Registration file is automatically cleaned up on normal shutdown via Drop; stale files from crashed processes are detected by checking /proc/<pid> and cleaned on next VMM startup
• vmm-cli.py gains ls-vmm and switch-vmm <id> commands for multi-instance management
• Auto-discovery: when no explicit --url is given, CLI auto-connects to the active or sole running VMM instance

Test plan

• Two TCP VMM instances: ls-vmm shows both with correct ports
• switch-vmm with prefix match, * marker, and lsvm auto-connects to selected instance
• UDS listener (unix:./vmm.sock): discovery file, path resolution, auto-connect all work
• Normal exit (SIGTERM): Drop removes registration file
• Abnormal exit (SIGKILL): stale file remains but ls-vmm filters by /proc/<pid> check
• cargo fmt, cargo clippy clean