Make ModelRelay a first-class provider

[shanev]

Summary

• add explicit ModelRelay provider selection and mr_sk_* key detection
• support MODELRELAY_API_KEY and MODELRELAY_BASE_URL in the CLI and embedded clients
• import existing ModelRelay keys during login setup and document all supported paths
• release as 0.26.0

Verification

• uv run pytest -q (1278 passed, 3 skipped, 2 subtests passed)
• uv run ruff check src tests
• uv run ruff format --check src tests
• wheel and sdist build plus isolated wheel install
• PyPI publish dry-run
• packaged CLI root and subcall smoke tests for environment, prefix detection, and explicit provider selection