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/workspace/util.go

@@ -0,0 +1,20 @@
+package workspace
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ShellQuote returns value single-quoted for bash, escaping embedded quotes.
+func ShellQuote(value string) string {
+	return "'" + strings.ReplaceAll(value, "'", `'"'"'`) + "'"
+}
+
+// FormatStepError wraps err with an action label and trimmed on-guest log.
+func FormatStepError(action string, err error, log string) error {
+	log = strings.TrimSpace(log)
+	if log == "" {
+		return fmt.Errorf("%s: %w", action, err)
+	}
+	return fmt.Errorf("%s: %w: %s", action, err, log)
+}