fix: hide write UI resources in read-only mode

[he-yufeng]

Fixes #2519

Summary

• pass read-only mode into MCP App UI resource registration
• keep get_me_ui available in read-only mode
• skip issue_write_ui and pr_write_ui when write tools are filtered out

To verify

• go test ./pkg/github -run TestRegisterUIResources -count=1
• go test ./cmd/github-mcp-server ./pkg/github
• go test ./internal/ghmcp ./pkg/github
• git diff --check