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>

internal/config/agents.go

@@ -27,6 +27,8 @@ const (
 	AgentAuggie AgentPreset = "auggie"
 	// AgentAmp is Sourcegraph AMP.
 	AgentAmp AgentPreset = "amp"
+	// AgentOpenCode is OpenCode multi-model CLI.
+	AgentOpenCode AgentPreset = "opencode"
 )
 
 // AgentPresetInfo contains the configuration details for an agent preset.
@@ -182,6 +184,25 @@ var builtinPresets = map[AgentPreset]*AgentPresetInfo{
 		SupportsHooks:       false,
 		SupportsForkSession: false,
 	},
+	AgentOpenCode: {
+		Name:    AgentOpenCode,
+		Command: "opencode",
+		Args:    []string{}, // No CLI flags needed, YOLO via OPENCODE_PERMISSION env
+		Env: map[string]string{
+			// Auto-approve all tool calls (equivalent to --dangerously-skip-permissions)
+			"OPENCODE_PERMISSION": `{"*":"allow"}`,
+		},
+		ProcessNames:        []string{"opencode", "node"}, // Runs as Node.js
+		SessionIDEnv:        "",                           // OpenCode manages sessions internally
+		ResumeFlag:          "",                           // No resume support yet
+		ResumeStyle:         "",
+		SupportsHooks:       true,  // Uses .opencode/plugin/gastown.js
+		SupportsForkSession: false,
+		NonInteractive: &NonInteractiveConfig{
+			Subcommand: "run",
+			OutputFlag: "--format json",
+		},
+	},
 }
 
 // Registry state with proper synchronization.