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/daemon/open_close_test.go

@@ -26,10 +26,9 @@ func TestCloseOnPartiallyInitialisedDaemon(t *testing.T) {
 			name: "only store + closing channel (early failure)",
 			build: func(t *testing.T) *Daemon {
 				return &Daemon{
-					store:    openDaemonStore(t),
-					closing:  make(chan struct{}),
-					sessions: newSessionRegistry(),
-					logger:   slog.New(slog.NewTextHandler(io.Discard, nil)),
+					store:   openDaemonStore(t),
+					closing: make(chan struct{}),
+					logger:  slog.New(slog.NewTextHandler(io.Discard, nil)),
 				}
 			},
 			verify: func(t *testing.T, d *Daemon) {
@@ -49,11 +48,10 @@ func TestCloseOnPartiallyInitialisedDaemon(t *testing.T) {
 					t.Fatalf("vmdns.New: %v", err)
 				}
 				return &Daemon{
-					store:    openDaemonStore(t),
-					closing:  make(chan struct{}),
-					sessions: newSessionRegistry(),
-					vmDNS:    server,
-					logger:   slog.New(slog.NewTextHandler(io.Discard, nil)),
+					store:   openDaemonStore(t),
+					closing: make(chan struct{}),
+					vmDNS:   server,
+					logger:  slog.New(slog.NewTextHandler(io.Discard, nil)),
 				}
 			},
 			verify: func(t *testing.T, d *Daemon) {
@@ -86,11 +84,10 @@ func TestCloseOnPartiallyInitialisedDaemon(t *testing.T) {
 // returns and also calls Close afterwards, both paths must survive.
 func TestCloseIdempotentUnderConcurrency(t *testing.T) {
 	d := &Daemon{
-		store:    openDaemonStore(t),
-		closing:  make(chan struct{}),
-		sessions: newSessionRegistry(),
-		logger:   slog.New(slog.NewTextHandler(io.Discard, nil)),
-		config:   model.DaemonConfig{BridgeName: ""},
+		store:   openDaemonStore(t),
+		closing: make(chan struct{}),
+		logger:  slog.New(slog.NewTextHandler(io.Discard, nil)),
+		config:  model.DaemonConfig{BridgeName: ""},
 	}
 
 	var count atomic.Int32