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/guest_ssh.go

@@ -0,0 +1,35 @@
+package daemon
+
+import (
+	"context"
+	"io"
+	"os"
+	"time"
+
+	"banger/internal/guest"
+)
+
+// guestSSHClient is the narrow guest-SSH surface the daemon uses for
+// workspace prepare / export and ad-hoc guest interactions.
+type guestSSHClient interface {
+	Close() error
+	RunScript(context.Context, string, io.Writer) error
+	RunScriptOutput(context.Context, string) ([]byte, error)
+	UploadFile(context.Context, string, os.FileMode, []byte, io.Writer) error
+	StreamTar(context.Context, string, string, io.Writer) error
+	StreamTarEntries(context.Context, string, []string, string, io.Writer) error
+}
+
+func (d *Daemon) waitForGuestSSH(ctx context.Context, address string, interval time.Duration) error {
+	if d != nil && d.guestWaitForSSH != nil {
+		return d.guestWaitForSSH(ctx, address, d.config.SSHKeyPath, interval)
+	}
+	return guest.WaitForSSH(ctx, address, d.config.SSHKeyPath, d.layout.KnownHostsPath, interval)
+}
+
+func (d *Daemon) dialGuest(ctx context.Context, address string) (guestSSHClient, error) {
+	if d != nil && d.guestDial != nil {
+		return d.guestDial(ctx, address, d.config.SSHKeyPath)
+	}
+	return guest.Dial(ctx, address, d.config.SSHKeyPath, d.layout.KnownHostsPath)
+}