fix(config): implement role_agents support in BuildStartupCommand (#19) (#456)

* fix(config): implement role_agents support in BuildStartupCommand

The role_agents field in TownSettings and RigSettings existed but was
not being used by the startup command builders. All services fell back
to the default agent instead of using role-specific agent assignments.

Changes:
- BuildStartupCommand now extracts GT_ROLE from envVars and uses
  ResolveRoleAgentConfig() for role-based agent selection
- BuildStartupCommandWithAgentOverride follows the same pattern when
  no explicit override is provided
- refinery/manager.go uses ResolveRoleAgentConfig with constants
- cmd/start.go uses ResolveRoleAgentConfig with constants
- Updated comments from hardcoded agent name to generic "agent"
- Added ValidateAgentConfig() to check agent exists and binary is in PATH
- Added lookupAgentConfigIfExists() helper for validation
- ResolveRoleAgentConfig now warns to stderr and falls back to default
  if configured agent is invalid or binary is missing

Resolution priority (now working):
1. Explicit --agent override
2. Rig's role_agents[role] (validated)
3. Town's role_agents[role] (validated)
4. Rig's agent setting
5. Town's default_agent
6. Hardcoded default fallback

Adds tests for:
- TestBuildStartupCommand_UsesRoleAgentsFromTownSettings
- TestBuildStartupCommand_RigRoleAgentsOverridesTownRoleAgents
- TestBuildAgentStartupCommand_UsesRoleAgents
- TestValidateAgentConfig
- TestResolveRoleAgentConfig_FallsBackOnInvalidAgent

Fixes: role_agents configuration not being applied to services

* fix(config): add GT_ROOT to BuildStartupCommandWithAgentOverride

- Fixes missing GT_ROOT and GT_SESSION_ID_ENV exports in
  BuildStartupCommandWithAgentOverride, matching BuildStartupCommand behavior
- Adds test for override priority over role_agents
- Adds test verifying GT_ROOT is included in command

This addresses the Greptile review comment about agents started with
an override not having access to town-level resources.

Co-authored-by: Steve Yegge <steve.yegge@gmail.com>

internal/cmd/start.go

@@ -381,10 +381,10 @@ func startConfiguredCrew(t *tmux.Tmux, rigs []*rig.Rig, townRoot string, mu *syn
 func startOrRestartCrewMember(t *tmux.Tmux, r *rig.Rig, crewName, townRoot string) (msg string, started bool) {
 	sessionID := crewSessionName(r.Name, crewName)
 	if running, _ := t.HasSession(sessionID); running {
-		// Session exists - check if Claude is still running
-		agentCfg := config.ResolveAgentConfig(townRoot, r.Path)
+		// Session exists - check if agent is still running
+		agentCfg := config.ResolveRoleAgentConfig(constants.RoleCrew, townRoot, r.Path)
 		if !t.IsAgentRunning(sessionID, config.ExpectedPaneCommands(agentCfg)...) {
-			// Claude has exited, restart it
+			// Agent has exited, restart it
 			// Build startup beacon for predecessor discovery via /resume
 			address := fmt.Sprintf("%s/crew/%s", r.Name, crewName)
 			beacon := session.FormatStartupNudge(session.StartupNudgeConfig{
@@ -392,11 +392,11 @@ func startOrRestartCrewMember(t *tmux.Tmux, r *rig.Rig, crewName, townRoot strin
 				Sender:    "human",
 				Topic:     "restart",
 			})
-			claudeCmd := config.BuildCrewStartupCommand(r.Name, crewName, r.Path, beacon)
-			if err := t.SendKeys(sessionID, claudeCmd); err != nil {
+			agentCmd := config.BuildCrewStartupCommand(r.Name, crewName, r.Path, beacon)
+			if err := t.SendKeys(sessionID, agentCmd); err != nil {
 				return fmt.Sprintf("  %s %s/%s restart failed: %v\n", style.Dim.Render("○"), r.Name, crewName, err), false
 			}
-			return fmt.Sprintf("  %s %s/%s Claude restarted\n", style.Bold.Render("✓"), r.Name, crewName), true
+			return fmt.Sprintf("  %s %s/%s agent restarted\n", style.Bold.Render("✓"), r.Name, crewName), true
 		}
 		return fmt.Sprintf("  %s %s/%s already running\n", style.Dim.Render("○"), r.Name, crewName), false
 	}