feat: Add rig-level custom agent support (#12)

* feat: Add rig-level custom agent support

Implement rig-level custom agent configuration support to enable per-rig
agent definitions in <rig>/settings/config.json, following the same pattern as
town-level agents in settings/config.json.

Changes:
- Added RigSettings.Agents field to internal/config/types.go
- Added DefaultRigAgentRegistryPath() and LoadRigAgentRegistry() functions to internal/config/agents.go
- Updated ResolveAgentConfigWithOverride() to accept and pass rigSettings parameter
- Updated GetRuntimeCommandWithAgentOverride() to use rigSettings when available
- Updated GetRuntimeCommandWithPromptAndAgentOverride() to use rigSettings
- Updated all Build*WithOverride functions to pass rigSettings

This fixes the issue where rig-level agent settings were loaded but
ignored by lookupAgentConfig, enabling per-rig custom agents for
polecats and crew members.

* test: Add rig-level custom agent tests

Added comprehensive unit tests for rig agent registry functions:
- TestDefaultRigAgentRegistryPath: verifies path construction
- TestLoadRigAgentRegistry: verifies file loading and JSON parsing
- TestLookupAgentConfigWithRigSettings: verifies agent lookup priority (rig > town > builtin)

Added placeholder integration test for future CI/CD setup.

* initial commit

* fix: resolve compilation errors in rig-level custom agent support

- Add missing RigAgentRegistryPath function (alias for DefaultRigAgentRegistryPath)
- Restore ResolveAgentConfigWithOverride function that was incorrectly removed
- Fix ResolveAgentConfig to return single value (not triple)
- Add initRegistryLocked() call to LoadRigAgentRegistry to prevent nil panic
- Fix DefaultRigAgentRegistryPath to use rigPath directly (not parent dir)
- Fix test file syntax errors (remove EOF artifacts)
- Fix test parameter order for lookupAgentConfig calls
- Fix test expectations to match correct custom agent override behavior

* test: implement rig-level custom agent integration test

- Add stub agent script that simulates AI agent with Q&A capability
- Test ResolveAgentConfig correctly picks up rig-level agents
- Test BuildPolecatStartupCommand includes custom agent command
- Test ResolveAgentConfigWithOverride respects rig agents
- Test rig agents override town agents with same name
- Add tmux integration test that spawns session and verifies output
- Stub agent echoes 'STUB_AGENT_STARTED' and handles ping/pong Q&A
- All tests pass including real tmux session verification

* docs: add OpenCode custom agent example to reference

- Show settings/agents.json format for advanced configs
- Include OpenCode example with session resume flags
- Document OPENCODE_PERMISSION env var for autonomous mode

* fix: improve rig-level agent support with docs and test fixes

- Add rig-level agent documentation to reference.md
- Document agent resolution order (rig → town → built-in)
- Deduplicate LoadAgentRegistry/LoadRigAgentRegistry into shared helper
- Fix test isolation in TestLoadRigAgentRegistry
- Fix nil pointer dereference in test assertions (use t.Fatal not t.Error)

internal/config/agents.go

@@ -215,16 +215,11 @@ func ensureRegistry() {
 	initRegistryLocked()
 }
 
-// LoadAgentRegistry loads agent definitions from a JSON file and merges with built-ins.
-// User-defined agents override built-in presets with the same name.
-// This function caches loaded paths to avoid redundant file reads.
-func LoadAgentRegistry(path string) error {
-	registryMu.Lock()
-	defer registryMu.Unlock()
-
+// loadAgentRegistryFromPath loads agent definitions from a JSON file and merges with built-ins.
+// Caller must hold registryMu write lock.
+func loadAgentRegistryFromPathLocked(path string) error {
 	initRegistryLocked()
 
-	// Check if already loaded from this path
 	if loadedPaths[path] {
 		return nil
 	}
@@ -232,8 +227,8 @@ func LoadAgentRegistry(path string) error {
 	data, err := os.ReadFile(path) //nolint:gosec // G304: path is from config
 	if err != nil {
 		if os.IsNotExist(err) {
-			loadedPaths[path] = true // Mark as "loaded" (no file)
-			return nil               // No custom config, use built-ins only
+			loadedPaths[path] = true
+			return nil
 		}
 		return err
 	}
@@ -243,7 +238,6 @@ func LoadAgentRegistry(path string) error {
 		return err
 	}
 
-	// Merge user-defined agents (override built-ins)
 	for name, preset := range userRegistry.Agents {
 		preset.Name = AgentPreset(name)
 		globalRegistry.Agents[name] = preset
@@ -253,12 +247,41 @@ func LoadAgentRegistry(path string) error {
 	return nil
 }
 
+// LoadAgentRegistry loads agent definitions from a JSON file and merges with built-ins.
+// User-defined agents override built-in presets with the same name.
+// This function caches loaded paths to avoid redundant file reads.
+func LoadAgentRegistry(path string) error {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+	return loadAgentRegistryFromPathLocked(path)
+}
+
 // DefaultAgentRegistryPath returns the default path for agent registry.
 // Located alongside other town settings.
 func DefaultAgentRegistryPath(townRoot string) string {
 	return filepath.Join(townRoot, "settings", "agents.json")
 }
 
+// DefaultRigAgentRegistryPath returns the default path for rig-level agent registry.
+// Located in <rig>/settings/agents.json.
+func DefaultRigAgentRegistryPath(rigPath string) string {
+	return filepath.Join(rigPath, "settings", "agents.json")
+}
+
+// RigAgentRegistryPath returns the path for rig-level agent registry.
+// Alias for DefaultRigAgentRegistryPath for consistency with other path functions.
+func RigAgentRegistryPath(rigPath string) string {
+	return DefaultRigAgentRegistryPath(rigPath)
+}
+
+// LoadRigAgentRegistry loads agent definitions from a rig-level JSON file and merges with built-ins.
+// This function works similarly to LoadAgentRegistry but for rig-level configurations.
+func LoadRigAgentRegistry(path string) error {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+	return loadAgentRegistryFromPathLocked(path)
+}
+
 // GetAgentPreset returns the preset info for a given agent name.
 // Returns nil if the preset is not found.
 func GetAgentPreset(name AgentPreset) *AgentPresetInfo {