DOC/TEST: add external agent integration walkthrough

[biru-codeastromer]

Summary

Closes #419.

This PR adds a small but concrete workflow artifact for integrating sktime-mcp from an external agent codebase.

What changed

• add a new runnable example: examples/08_external_agent_integration.py
• add a smoke test for that example
• extend the user guide with:
  • editable-install guidance for external agent projects
  • a PYTHONPATH fallback for temporary source-tree integrations
  • a concrete evaluate_estimator usage example with the current MCP contract
  • guidance to inspect returned handle metadata before model selection

Why this matters

This contribution maps directly to real MCP adoption work:

• it helps users wire sktime-mcp into their own agent projects
• it makes setup less error-prone
• it gives agents and developers a tested example for combining custom-data loading, forecasting, and evaluation

Validation

• .venv/bin/ruff check examples/08_external_agent_integration.py tests/test_external_agent_integration_example.py
• .venv/bin/ruff format --check examples/08_external_agent_integration.py tests/test_external_agent_integration_example.py
• .venv/bin/python examples/08_external_agent_integration.py
• .venv/bin/pytest tests/test_external_agent_integration_example.py -q
• .venv/bin/pytest -q