fix: resolve all compilation errors and clippy warnings

This commit fixes 12 categories of errors across the codebase:

- Fix owlen-mcp-llm-server build target conflict by renaming lib.rs to main.rs
- Resolve ambiguous glob re-exports in owlen-core by using explicit exports
- Add Default derive to MockMcpClient and MockProvider test utilities
- Remove unused imports from owlen-core test files
- Fix needless borrows in test file arguments
- Improve Config initialization style in mode_tool_filter tests
- Make AgentExecutor::parse_response public for testing
- Remove non-existent max_tool_calls field from AgentConfig usage
- Fix AgentExecutor::new calls to use correct 3-argument signature
- Fix AgentResult field access in agent tests
- Use Debug formatting instead of Display for AgentResult
- Remove unnecessary default() calls on unit structs

All changes ensure the project compiles cleanly with:
- cargo check --all-targets ✓
- cargo clippy --all-targets -- -D warnings ✓
- cargo test --no-run ✓

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/owlen-core/src/mcp.rs

@@ -149,14 +149,9 @@ pub mod test_utils {
     use super::*;
 
     /// Mock MCP client for testing
+    #[derive(Default)]
     pub struct MockMcpClient;
 
-    impl MockMcpClient {
-        pub fn new() -> Self {
-            Self
-        }
-    }
-
     #[async_trait]
     impl McpClient for MockMcpClient {
         async fn list_tools(&self) -> Result<Vec<McpToolDescriptor>> {