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/features/claude-code-agent-loader/agent-definitions-loader.ts

@@ -28,7 +28,8 @@ export function parseMarkdownAgentFile(filePath: string, scope: AgentScope): Loa
     const content = readFileSync(filePath, "utf-8")
     const { data, body } = parseFrontmatter<AgentFrontmatter>(content)
 
-    const agentName = basename(filePath, ".md")
+    const fileName = basename(filePath)
+    const agentName = fileName.replace(/\.md$/i, "")
     const name = data.name || agentName
     const originalDescription = data.description || ""