Extract session subpackage with pure guest-session helpers

Moves the stateless parts of the guest-session subsystem into
internal/daemon/session:

- consts (BackendSSH, attach/transport kinds, StateRoot, LogTailLineDefault)
- StateSnapshot plus ParseState / InspectStateFromDir / ApplyStateSnapshot / StateChanged
- 10 on-guest path helpers (StateDir, StdoutLogPath, StdinPipePath, …)
- 3 bash script generators (Script, InspectScript, SignalScript)
- small utilities (ShellQuote, ExitCode, CloneStringMap, TailFileContent,
  ProcessAlive + syscallKill test seam, FormatStepError)
- launch helpers (DefaultName, DefaultCWD, FailLaunch,
  NormalizeRequiredCommands, CWDPreflightScript, CommandPreflightScript,
  AttachInputCommand, AttachTailCommand, EnvLines)

Callers inside the daemon package import the new package under the
alias "sess" to avoid colliding with the local `session model.GuestSession`
variables threaded through the orchestrator code. guest_sessions.go
shrinks from 616 → 156 LOC; session_stream.go, session_attach.go,
session_lifecycle.go, workspace.go, and guest_sessions_test.go rewire to
the exported names.

The orchestrator methods (StartGuestSession, BeginGuestSessionAttach,
SendToGuestSession, GuestSessionLogs, refresh/inspect, sessionRegistry,
guestSessionController) stay on *Daemon. Full Manager-style extraction
would need prerequisite phases (operation protocol, workdisk helpers),
mirroring Phase 4a's trade-off.

All tests green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/daemon/session_attach.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"banger/internal/api"
+	sess "banger/internal/daemon/session"
 	"banger/internal/guest"
 	"banger/internal/model"
 	"banger/internal/sessionstream"
@@ -56,7 +57,7 @@ func (d *Daemon) BeginGuestSessionAttach(ctx context.Context, params api.GuestSe
 	return api.GuestSessionAttachBeginResult{
 		Session:         session,
 		AttachID:        attachID,
-		TransportKind:   guestSessionTransportUnixSocket,
+		TransportKind:   sess.TransportUnixSocket,
 		TransportTarget: socketPath,
 		SocketPath:      socketPath,
 		StreamFormat:    sessionstream.FormatV1,
@@ -86,7 +87,7 @@ func (d *Daemon) waitForGuestSessionExit(id string, controller *guestSessionCont
 	now := model.Now()
 	updated.UpdatedAt = now
 	updated.EndedAt = now
-	if exitCode, ok := guestSessionExitCode(err); ok {
+	if exitCode, ok := sess.ExitCode(err); ok {
 		updated.ExitCode = &exitCode
 		if exitCode == 0 {
 			updated.Status = model.GuestSessionStatusExited
@@ -165,16 +166,16 @@ func (d *Daemon) attachGuestSessionBridge(session model.GuestSession, controller
 		return fmt.Errorf("vm %q is not running", vm.Name)
 	}
 	address := net.JoinHostPort(vm.Runtime.GuestIP, "22")
-	stdinStream, err := d.openGuestSessionAttachStream(address, guestSessionAttachInputCommand(session.ID))
+	stdinStream, err := d.openGuestSessionAttachStream(address, sess.AttachInputCommand(session.ID))
 	if err != nil {
 		return fmt.Errorf("open guest session stdin stream: %w", err)
 	}
-	stdoutStream, err := d.openGuestSessionAttachStream(address, guestSessionAttachTailCommand(session.StdoutLogPath))
+	stdoutStream, err := d.openGuestSessionAttachStream(address, sess.AttachTailCommand(session.StdoutLogPath))
 	if err != nil {
 		_ = stdinStream.Close()
 		return fmt.Errorf("open guest session stdout stream: %w", err)
 	}
-	stderrStream, err := d.openGuestSessionAttachStream(address, guestSessionAttachTailCommand(session.StderrLogPath))
+	stderrStream, err := d.openGuestSessionAttachStream(address, sess.AttachTailCommand(session.StderrLogPath))
 	if err != nil {
 		_ = stdinStream.Close()
 		_ = stdoutStream.Close()