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/api/types.go

@@ -122,70 +122,6 @@ type VMPortsResult struct {
 	Ports   []VMPort `json:"ports"`
 }
 
-type GuestSessionStartParams struct {
-	VMIDOrName       string            `json:"vm_id_or_name"`
-	Name             string            `json:"name,omitempty"`
-	Command          string            `json:"command"`
-	Args             []string          `json:"args,omitempty"`
-	CWD              string            `json:"cwd,omitempty"`
-	Env              map[string]string `json:"env,omitempty"`
-	StdinMode        string            `json:"stdin_mode,omitempty"`
-	Tags             map[string]string `json:"tags,omitempty"`
-	RequiredCommands []string          `json:"required_commands,omitempty"`
-}
-
-type GuestSessionRefParams struct {
-	VMIDOrName      string `json:"vm_id_or_name"`
-	SessionIDOrName string `json:"session_id_or_name"`
-}
-
-type GuestSessionLogsParams struct {
-	VMIDOrName      string `json:"vm_id_or_name"`
-	SessionIDOrName string `json:"session_id_or_name"`
-	Stream          string `json:"stream,omitempty"`
-	TailLines       int    `json:"tail_lines,omitempty"`
-}
-
-type GuestSessionAttachBeginParams struct {
-	VMIDOrName      string `json:"vm_id_or_name"`
-	SessionIDOrName string `json:"session_id_or_name"`
-}
-
-type GuestSessionListResult struct {
-	Sessions []model.GuestSession `json:"sessions"`
-}
-
-type GuestSessionShowResult struct {
-	Session model.GuestSession `json:"session"`
-}
-
-type GuestSessionLogsResult struct {
-	Session model.GuestSession `json:"session"`
-	Stream  string             `json:"stream"`
-	Path    string             `json:"path,omitempty"`
-	Content string             `json:"content,omitempty"`
-}
-
-type GuestSessionAttachBeginResult struct {
-	Session         model.GuestSession `json:"session"`
-	AttachID        string             `json:"attach_id"`
-	TransportKind   string             `json:"transport_kind"`
-	TransportTarget string             `json:"transport_target"`
-	SocketPath      string             `json:"socket_path,omitempty"`
-	StreamFormat    string             `json:"stream_format"`
-}
-
-type GuestSessionSendParams struct {
-	VMIDOrName      string `json:"vm_id_or_name"`
-	SessionIDOrName string `json:"session_id_or_name"`
-	Payload         []byte `json:"payload"`
-}
-
-type GuestSessionSendResult struct {
-	Session      model.GuestSession `json:"session"`
-	BytesWritten int                `json:"bytes_written"`
-}
-
 type WorkspaceExportParams struct {
 	IDOrName   string `json:"id_or_name"`
 	GuestPath  string `json:"guest_path,omitempty"`