feat(config): add OpenCode as built-in agent preset (#861)

Add AgentOpenCode as a first-class built-in agent preset, similar to
Claude, Gemini, Codex, Cursor, Auggie, and AMP.

OpenCode preset configuration:
- Command: "opencode"
- Args: [] (uses Env for YOLO mode, no CLI flags needed)
- Env: OPENCODE_PERMISSION='{"*":"allow"}' for auto-approve
- ProcessNames: ["opencode", "node"] (runs as Node.js)
- SupportsHooks: true (uses .opencode/plugin/gastown.js)
- NonInteractive: run subcommand with --format json

Runtime defaults for opencode provider:
- ready_delay_ms: 8000 (delay-based detection for TUI)
- process_names: [opencode, node]
- instructions_file: AGENTS.md

This allows users to simply configure:
  role_agents:
    refinery: "opencode"

Instead of manually configuring agents.json and runtime settings.

Test coverage:
- TestOpenCodeAgentPreset: comprehensive preset validation
- TestOpenCodeProviderDefaults: runtime config defaults
- TestOpenCodeRuntimeConfigFromPreset: Env copying
- TestIsKnownPreset: includes opencode
- TestGetAgentPresetByName: opencode returns preset

Templates added:
- templates/agents/opencode.json.tmpl: agent config template
- templates/agents/opencode-models.json: model delay presets

Co-authored-by: Avyukth <subhrajit.makur@hotmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

templates/agents/opencode-models.json

@@ -0,0 +1,60 @@
+{
+  "version": 1,
+  "description": "OpenCode model presets with recommended delay settings for gastown integration",
+  "usage": "Use ready_delay_ms values when configuring runtime.tmux.ready_delay_ms in agents.json",
+  "models_api": "https://models.dev/api.json",
+  "models_api_note": "LLMs should fetch current model list from models_api. The 'models' below are fallback examples with gastown-specific delay recommendations.",
+  "models": {
+    "openai": {
+      "gpt-5.2": {
+        "description": "GPT-5.2 chat model",
+        "ready_delay_ms": 5000
+      },
+      "gpt-5.2-codex": {
+        "description": "GPT-5.2 Codex for code tasks",
+        "ready_delay_ms": 8000
+      },
+      "codex-1": {
+        "description": "Codex-1 extended context",
+        "ready_delay_ms": 10000
+      }
+    },
+    "google": {
+      "gemini-3-pro-high": {
+        "description": "Gemini 3 Pro High quality",
+        "ready_delay_ms": 6000
+      }
+    },
+    "xai": {
+      "grok-code-fast-1": {
+        "description": "Grok Code Fast",
+        "ready_delay_ms": 4000
+      }
+    },
+    "github-copilot": {
+      "gpt-5.2-codex": {
+        "description": "GitHub Copilot GPT-5.2 Codex",
+        "ready_delay_ms": 8000
+      }
+    },
+    "opencode": {
+      "glm-4.7-free": {
+        "description": "GLM 4.7 Free tier (may timeout, needs longer delay)",
+        "ready_delay_ms": 15000
+      },
+      "minimax-m2.1-free": {
+        "description": "MiniMax M2.1 Free tier",
+        "ready_delay_ms": 10000
+      },
+      "big-pickle": {
+        "description": "Big Pickle experimental",
+        "ready_delay_ms": 12000
+      }
+    }
+  },
+  "notes": {
+    "why_delay_based": "OpenCode TUI uses box-drawing characters that break prompt prefix matching. Delay-based detection is required.",
+    "free_tier_warning": "Free tier models may have longer cold start times. Increase delay if timeout errors occur.",
+    "default_delay": 8000
+  }
+}

templates/agents/opencode.json.tmpl

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://gastown.dev/schemas/agent-template-v1.json",
+  "_models_api": "https://models.dev/api.json",
+  "_note": "LLMs: fetch available models from _models_api to get current provider/model options",
+  "name": "opencode-{{.Model}}",
+  "description": "OpenCode agent using {{.Provider}}/{{.Model}}",
+  "command": "{{.OpenCodePath | default \"opencode\"}}",
+  "args": ["-m", "{{.Provider}}/{{.Model}}"],
+  "non_interactive": {
+    "subcommand": "run",
+    "output_flag": "--format json"
+  },
+  "runtime": {
+    "provider": "opencode",
+    "tmux": {
+      "ready_prompt_prefix": "",
+      "ready_delay_ms": {{.ReadyDelayMs | default 8000}},
+      "process_names": ["opencode", "node"]
+    }
+  },
+  "hooks": {
+    "provider": "opencode"
+  }
+}