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/fake_firecracker_test.go

@@ -0,0 +1,26 @@
+package daemon
+
+import (
+	"fmt"
+	"os/exec"
+	"testing"
+)
+
+// startFakeFirecracker launches a bash sleep-loop rewritten to match
+// the firecracker command line a real process would expose, so
+// reconcile / handle-cache paths that grep /proc/<pid>/cmdline accept
+// it as a firecracker process. Killed on test cleanup.
+func startFakeFirecracker(t *testing.T, apiSock string) *exec.Cmd {
+	t.Helper()
+	cmd := exec.Command("bash", "-lc", fmt.Sprintf("exec -a %q sleep 60", "firecracker --api-sock "+apiSock))
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("start fake firecracker: %v", err)
+	}
+	t.Cleanup(func() {
+		if cmd.Process != nil {
+			_ = cmd.Process.Kill()
+			_, _ = cmd.Process.Wait()
+		}
+	})
+	return cmd
+}