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/cli/printers.go

@@ -3,7 +3,6 @@ package cli
 import (
 	"encoding/json"
 	"fmt"
-	"io"
 	"os"
 	"sort"
 	"strings"
@@ -276,30 +275,6 @@ func printKernelCatalogTable(out anyWriter, entries []api.KernelCatalogEntry) er
 	return w.Flush()
 }
 
-// -- guest session printers -----------------------------------------
-
-func printGuestSessionSummary(out anyWriter, session model.GuestSession) error {
-	_, err := fmt.Fprintf(out, "%s\t%s\t%s\t%s\t%s\n", session.ID, session.Name, session.Status, session.Command, session.CWD)
-	return err
-}
-
-func printGuestSessionTable(out io.Writer, sessions []model.GuestSession) error {
-	tw := tabwriter.NewWriter(out, 0, 0, 2, ' ', 0)
-	if _, err := fmt.Fprintln(tw, "ID\tNAME\tSTATUS\tATTACH\tCOMMAND\tCWD"); err != nil {
-		return err
-	}
-	for _, session := range sessions {
-		attach := "no"
-		if session.Attachable {
-			attach = "yes"
-		}
-		if _, err := fmt.Fprintf(tw, "%s\t%s\t%s\t%s\t%s\t%s\n", shortID(session.ID), session.Name, session.Status, attach, session.Command, session.CWD); err != nil {
-			return err
-		}
-	}
-	return tw.Flush()
-}
-
 // -- doctor printer -------------------------------------------------
 
 func printDoctorReport(out anyWriter, report system.Report) error {