feat(agents): wire agent_definitions and opencode.json agents into precedence chain

- Modified agent-config-handler.ts to load and integrate both new agent sources
- Added loadAgentDefinitions() and readOpencodeConfigAgents() calls in loading phase
- Integrated both sources into agent precedence chains (both Sisyphus-enabled and disabled paths)
- Added detailed logging for new agent sources
- Added filtering logic to respect disabled_agents configuration
- Extended agent-config-handler.test.ts with 7 new integration tests
- All tests passing (18/18 integration, 65/65 loader suite)

Wave 3 of agent definitions enhancement complete.

src/features/claude-code-agent-loader/opencode-config-agents-reader.ts

@@ -9,6 +9,7 @@ import type { ClaudeCodeAgentConfig } from "./types"
 
 interface OpencodeConfigWithAgents {
   agents?: Record<string, unknown>
+  agent?: Record<string, unknown>
   agent_definitions?: string | string[]
 }
 
@@ -103,8 +104,10 @@ export function readOpencodeConfigAgents(directory: string): Record<string, Clau
 
       const configDir = path.dirname(configPath)
 
-      if (parseResult.data.agents && typeof parseResult.data.agents === "object") {
-        for (const [agentName, agentData] of Object.entries(parseResult.data.agents)) {
+      const agentsToLoad = parseResult.data.agents || parseResult.data.agent
+
+      if (agentsToLoad && typeof agentsToLoad === "object") {
+        for (const [agentName, agentData] of Object.entries(agentsToLoad)) {
           const converted = convertInlineAgent(agentData)
           if (converted) {
             result[agentName] = converted