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/cmd/up.go

@@ -140,7 +140,7 @@ func runUp(cmd *cobra.Command, args []string) error {
 		}
 
 		mgr := refinery.NewManager(r)
-		if err := mgr.Start(false); err != nil {
+		if err := mgr.Start(false, ""); err != nil {
 			if err == refinery.ErrAlreadyRunning {
 				printStatus(fmt.Sprintf("Refinery (%s)", rigName), true, mgr.SessionName())
 			} else {