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_stream.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"banger/internal/api"
+	sess "banger/internal/daemon/session"
 	"banger/internal/guest"
 	"banger/internal/model"
 	"banger/internal/system"
@@ -30,7 +31,7 @@ func (d *Daemon) GuestSessionLogs(ctx context.Context, params api.GuestSessionLo
 	}
 	tailLines := params.TailLines
 	if tailLines <= 0 {
-		tailLines = guestSessionLogTailLine
+		tailLines = sess.LogTailLineDefault
 	}
 	path := session.StdoutLogPath
 	if streamName == "stderr" {
@@ -76,9 +77,9 @@ func (d *Daemon) SendToGuestSession(ctx context.Context, params api.GuestSession
 	}
 	sendScript := fmt.Sprintf(
 		"set -euo pipefail\ncat %s >> %s\nrm -f %s\n",
-		guestShellQuote(tmpPath),
-		guestShellQuote(guestSessionStdinPipePath(session.ID)),
-		guestShellQuote(tmpPath),
+		sess.ShellQuote(tmpPath),
+		sess.ShellQuote(sess.StdinPipePath(session.ID)),
+		sess.ShellQuote(tmpPath),
 	)
 	var sendLog bytes.Buffer
 	if err := client.RunScript(ctx, sendScript, &sendLog); err != nil {
@@ -99,9 +100,9 @@ func (d *Daemon) readGuestSessionLog(ctx context.Context, vm model.VMRecord, ses
 			path = session.StderrLogPath
 		}
 		var output bytes.Buffer
-		script := fmt.Sprintf("set -euo pipefail\nif [ -f %s ]; then tail -n %d %s; fi\n", guestShellQuote(path), tailLines, guestShellQuote(path))
+		script := fmt.Sprintf("set -euo pipefail\nif [ -f %s ]; then tail -n %d %s; fi\n", sess.ShellQuote(path), tailLines, sess.ShellQuote(path))
 		if err := client.RunScript(ctx, script, &output); err != nil {
-			return "", formatGuestSessionStepError("read guest session log", err, output.String())
+			return "", sess.FormatStepError("read guest session log", err, output.String())
 		}
 		return output.String(), nil
 	}
@@ -114,6 +115,6 @@ func (d *Daemon) readGuestSessionLog(ctx context.Context, vm model.VMRecord, ses
 		return "", err
 	}
 	defer cleanup()
-	logPath := filepath.Join(workMount, guestSessionRelativeStateDir(session.ID), stream+".log")
-	return tailFileContent(logPath, tailLines)
+	logPath := filepath.Join(workMount, sess.RelativeStateDir(session.ID), stream+".log")
+	return sess.TailFileContent(logPath, tailLines)
 }