fix(agents): address cubic review findings on agent loader

- Case-insensitive .md extension stripping for agent name extraction
- Resolve project agent_definitions paths relative to config dir (.opencode/)
- Use getOpenCodeConfigDir() to respect OPENCODE_CONFIG_DIR/XDG_CONFIG_HOME
- First-write-wins semantics for both inline and definition-file agents
  so project-level agents always take precedence over global-level

src/plugin-config.ts

@@ -278,7 +278,7 @@ export function loadPluginConfig(
   if (projectConfig?.agent_definitions) {
     projectConfig.agent_definitions = resolveAgentDefinitionPaths(
       projectConfig.agent_definitions,
-      directory,
+      projectBasePath,
       directory
     )
   }