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)

docs/reference.md

@@ -361,13 +361,54 @@ gt config default-agent [name]    # Get or set town default agent
 
 **Built-in agents**: `claude`, `gemini`, `codex`, `cursor`, `auggie`, `amp`
 
-**Custom agents**: Define per-town in `mayor/town.json`:
+**Custom agents**: Define per-town via CLI or JSON:
 ```bash
 gt config agent set claude-glm "claude-glm --model glm-4"
 gt config agent set claude "claude-opus"  # Override built-in
 gt config default-agent claude-glm       # Set default
 ```
 
+**Advanced agent config** (`settings/agents.json`):
+```json
+{
+  "version": 1,
+  "agents": {
+    "opencode": {
+      "command": "opencode",
+      "args": [],
+      "resume_flag": "--session",
+      "resume_style": "flag",
+      "non_interactive": {
+        "subcommand": "run",
+        "output_flag": "--format json"
+      }
+    }
+  }
+}
+```
+
+**Rig-level agents** (`<rig>/settings/config.json`):
+```json
+{
+  "type": "rig-settings",
+  "version": 1,
+  "agent": "opencode",
+  "agents": {
+    "opencode": {
+      "command": "opencode",
+      "args": ["--session"]
+    }
+  }
+}
+```
+
+**Agent resolution order**: rig-level → town-level → built-in presets.
+
+For OpenCode autonomous mode, set env var in your shell profile:
+```bash
+export OPENCODE_PERMISSION='{"*":"allow"}'
+```
+
 ### Rig Management
 
 ```bash