remove vm session feature

Cuts the daemon-managed guest-session machinery (start/list/show/
logs/stop/kill/attach/send). The feature shipped aimed at agent-
orchestration workflows (programmatic stdin piping into a long-lived
guest process) that aren't driving any concrete user today, and the
~2.3K LOC of daemon surface area — attach bridge, FIFO keepalive,
controller registry, sessionstream framing, SQLite persistence — was
locking in an API we'd have to keep through v0.1.0.

Anything session-flavoured that people actually need today can be
done with `vm ssh + tmux` or `vm run -- cmd`.

Deleted:
- internal/cli/commands_vm_session.go
- internal/daemon/{guest_sessions,session_lifecycle,session_attach,session_stream,session_controller}.go
- internal/daemon/session/ (guest-session helpers package)
- internal/sessionstream/ (framing package)
- internal/daemon/guest_sessions_test.go
- internal/store/guest_session_test.go
- GuestSession* types from internal/{api,model}
- Store UpsertGuestSession/GetGuestSession/ListGuestSessionsByVM/DeleteGuestSession + scanner helpers
- guest.session.* RPC dispatch entries
- 5 CLI session tests, 2 completion tests, 2 printer tests

Extracted:
- ShellQuote + FormatStepError lifted to internal/daemon/workspace/util.go
  (only non-session consumer); workspace package now self-contained
- internal/daemon/guest_ssh.go keeps guestSSHClient + dialGuest +
  waitForGuestSSH — still used by workspace prepare/export
- internal/daemon/fake_firecracker_test.go preserves the test helper
  that used to live in guest_sessions_test.go

Store schema: CREATE TABLE guest_sessions and its column migrations
removed. Existing dev DBs keep an orphan table (harmless, pre-v0.1.0).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/model/types.go

@@ -34,23 +34,6 @@ const (
 	VMStateError   VMState = "error"
 )
 
-type GuestSessionStatus string
-
-const (
-	GuestSessionStatusStarting GuestSessionStatus = "starting"
-	GuestSessionStatusRunning  GuestSessionStatus = "running"
-	GuestSessionStatusExited   GuestSessionStatus = "exited"
-	GuestSessionStatusFailed   GuestSessionStatus = "failed"
-	GuestSessionStatusStopping GuestSessionStatus = "stopping"
-)
-
-type GuestSessionStdinMode string
-
-const (
-	GuestSessionStdinClosed GuestSessionStdinMode = "closed"
-	GuestSessionStdinPipe   GuestSessionStdinMode = "pipe"
-)
-
 type DaemonConfig struct {
 	LogLevel            string
 	FirecrackerBin      string
@@ -176,37 +159,6 @@ type VMSetRequest struct {
 	NATEnabled        *bool
 }
 
-type GuestSession struct {
-	ID            string                `json:"id"`
-	VMID          string                `json:"vm_id"`
-	Name          string                `json:"name"`
-	Backend       string                `json:"backend"`
-	AttachBackend string                `json:"attach_backend,omitempty"`
-	AttachMode    string                `json:"attach_mode,omitempty"`
-	Command       string                `json:"command"`
-	Args          []string              `json:"args,omitempty"`
-	CWD           string                `json:"cwd,omitempty"`
-	Env           map[string]string     `json:"env,omitempty"`
-	StdinMode     GuestSessionStdinMode `json:"stdin_mode,omitempty"`
-	Status        GuestSessionStatus    `json:"status"`
-	ExitCode      *int                  `json:"exit_code,omitempty"`
-	GuestPID      int                   `json:"guest_pid,omitempty"`
-	GuestStateDir string                `json:"guest_state_dir,omitempty"`
-	StdoutLogPath string                `json:"stdout_log_path,omitempty"`
-	StderrLogPath string                `json:"stderr_log_path,omitempty"`
-	Tags          map[string]string     `json:"tags,omitempty"`
-	LastError     string                `json:"last_error,omitempty"`
-	Attachable    bool                  `json:"attachable"`
-	Reattachable  bool                  `json:"reattachable"`
-	LaunchStage   string                `json:"launch_stage,omitempty"`
-	LaunchMessage string                `json:"launch_message,omitempty"`
-	LaunchRawLog  string                `json:"launch_raw_log,omitempty"`
-	CreatedAt     time.Time             `json:"created_at"`
-	StartedAt     time.Time             `json:"started_at,omitempty"`
-	UpdatedAt     time.Time             `json:"updated_at"`
-	EndedAt       time.Time             `json:"ended_at,omitempty"`
-}
-
 type WorkspacePrepareMode string
 
 const (