feat(refinery,boot): add --agent flag for model selection (#469)

* feat(refinery,boot): add --agent flag for model selection (hq-7d5m)

Add --agent flag to gt refinery start/attach/restart and gt boot spawn
commands for consistent model selection across all agent launch points.

Implementation follows the existing pattern from gt deacon start:
- Add StringVar flag for agent alias
- Pass override to Manager/Boot via SetAgentOverride()
- Use BuildAgentStartupCommandWithAgentOverride when override is set

Files affected:
- cmd/gt/refinery.go: add flags to start/attach/restart commands
- internal/refinery/manager.go: add SetAgentOverride and use in Start()
- cmd/gt/boot.go: add flag to spawn command
- internal/boot/boot.go: add SetAgentOverride and use in spawnTmux()

Closes #438

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor(refinery,boot): use parameter-passing pattern for --agent flag

Address PR review feedback:

1. ADD TESTS: Add tests for --agent flag existence following witness_test.go pattern
   - internal/cmd/refinery_test.go: tests for start/attach/restart
   - internal/cmd/boot_test.go: test for spawn

2. ALIGN PATTERN: Change from setter pattern to parameter-passing pattern
   - Manager.Start(foreground, agentOverride) instead of SetAgentOverride + Start
   - Boot.Spawn(agentOverride) instead of SetAgentOverride + Spawn
   - Matches witness.go style: Start(foreground bool, agentOverride string, ...)

Updated all callers to pass empty string for default agent:
- internal/daemon/daemon.go
- internal/cmd/rig.go
- internal/cmd/start.go
- internal/cmd/up.go

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: furiosa <will@saults.io>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/boot/boot.go

@@ -41,11 +41,11 @@ type Status struct {
 
 // Boot manages the Boot watchdog lifecycle.
 type Boot struct {
-	townRoot   string
-	bootDir    string // ~/gt/deacon/dogs/boot/
-	deaconDir  string // ~/gt/deacon/
-	tmux       *tmux.Tmux
-	degraded   bool
+	townRoot  string
+	bootDir   string // ~/gt/deacon/dogs/boot/
+	deaconDir string // ~/gt/deacon/
+	tmux      *tmux.Tmux
+	degraded  bool
 }
 
 // New creates a new Boot manager.
@@ -145,7 +145,8 @@ func (b *Boot) LoadStatus() (*Status, error) {
 // Spawn starts Boot in a fresh tmux session.
 // Boot runs the mol-boot-triage molecule and exits when done.
 // In degraded mode (no tmux), it runs in a subprocess.
-func (b *Boot) Spawn() error {
+// The agentOverride parameter allows specifying an agent alias to use instead of the town default.
+func (b *Boot) Spawn(agentOverride string) error {
 	if b.IsRunning() {
 		return fmt.Errorf("boot is already running")
 	}
@@ -155,11 +156,11 @@ func (b *Boot) Spawn() error {
 		return b.spawnDegraded()
 	}
 
-	return b.spawnTmux()
+	return b.spawnTmux(agentOverride)
 }
 
 // spawnTmux spawns Boot in a tmux session.
-func (b *Boot) spawnTmux() error {
+func (b *Boot) spawnTmux(agentOverride string) error {
 	// Kill any stale session first
 	if b.IsSessionAlive() {
 		_ = b.tmux.KillSession(SessionName)
@@ -170,9 +171,18 @@ func (b *Boot) spawnTmux() error {
 		return fmt.Errorf("ensuring boot dir: %w", err)
 	}
 
-	// Build startup command first
+	// Build startup command with optional agent override
 	// The "gt boot triage" prompt tells Boot to immediately start triage (GUPP principle)
-	startCmd := config.BuildAgentStartupCommand("boot", "deacon-boot", "", "gt boot triage")
+	var startCmd string
+	if agentOverride != "" {
+		var err error
+		startCmd, err = config.BuildAgentStartupCommandWithAgentOverride("boot", "deacon-boot", "", "gt boot triage", agentOverride)
+		if err != nil {
+			return fmt.Errorf("building startup command with agent override: %w", err)
+		}
+	} else {
+		startCmd = config.BuildAgentStartupCommand("boot", "deacon-boot", "", "gt boot triage")
+	}
 
 	// Create session with command directly to avoid send-keys race condition.
 	// See: https://github.com/anthropics/gastown/issues/280