Sub-issue of #56 (G2 of TechDevGroup/devagentic#203).

Scope

• fetch_url(url) MCP tool — wraps devagentic's fetchUrl resolver per test: add unit tests for trajectory_compressor NousResearch/hermes-agent#203 §1.2.

Acceptance

• Tool registered under mcp_serve.py::_register_devagentic_mutation_tools.
• Confirms devagentic exposes fetchUrl (if not, file devagentic-side sub-issue first).
• Unit tests covering URL validation, response shape, error paths.