ci: add monthly Python 3.15 dependency canary

[stefan-jansen]

Summary

• run a monthly, non-blocking Ubuntu canary against Python 3.15 and the current core dependency constraints
• keep the canary separate from pull-request and supported-version qualification
• retain focused Data source coverage for core contracts, storage paths, and AQR factor handling
• confirm that all tracked public agent guides already satisfy the ecosystem standard, match current exports and workflows, and need no source edit

Verification

• actionlint .github/workflows/*.yml
• uv run ruff check src tests scripts
• uv run ruff format --check src tests scripts
• uv run ty check
• uv run pytest tests -q -ra (3,649 passed, 280 deselected)
• uv run pytest tests -q -ra -W error::ResourceWarning (3,649 passed, 280 deselected)
• uv run mkdocs build --strict
• uv build
• pre-commit run --all-files

The fresh Python 3.15 source probe remains non-blocking and reproduces the accepted dependency wait. Issue #37 stays open for the final cross-platform qualification trigger.

Closes #58

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