feat(config): Add multi-agent support with pluggable registry

Implements agent abstraction layer to support multiple AI coding agents.

Built-in presets (E2E tested):
- Claude Code (default)
- Gemini CLI
- OpenAI Codex

Key changes:
- Add AgentRegistry with built-in presets and custom agent support
- Add TownSettings with default_agent and custom agents map
- Add Agent field to RigSettings for per-rig agent selection
- Update ResolveAgentConfig for hierarchical config resolution
- Update spawn paths to use configured agent instead of hardcoded claude

Configuration hierarchy (first match wins):
1. Rig's Runtime config (backwards compat)
2. Rig's Agent -> custom agents -> built-in presets
3. Town's default_agent setting
4. Fallback to Claude

Additional agents (aider, opencode, etc.) can be added via config file:
settings/agents.json

Addresses Issue #10: Agent Agnostic Engine with Multi-provider support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/cmd/crew_at.go

@@ -183,10 +183,11 @@ func runCrewAt(cmd *cobra.Command, args []string) error {
 
 	// Check if we're already in the target session
 	if isInTmuxSession(sessionID) {
-		// We're in the session at a shell prompt - just start Claude directly
-		// Pass "gt prime" as initial prompt so Claude loads context immediately
-		fmt.Printf("Starting Claude in current session...\n")
-		return execClaude("gt prime")
+		// We're in the session at a shell prompt - just start the agent directly
+		// Pass "gt prime" as initial prompt so it loads context immediately
+		agentCfg := config.ResolveAgentConfig(townRoot, r.Path)
+		fmt.Printf("Starting %s in current session...\n", agentCfg.Command)
+		return execAgent(agentCfg, "gt prime")
 	}
 
 	// If inside tmux (but different session), don't switch - just inform user