docs: adopt shared contribution policy in Backtest

[stefan-jansen]

Owning issue

Closes #134

Outcome

Adds a Backtest CONTRIBUTING.md linked to the published ML4T policy and makes it reachable from the README. The existing bug, feature, and documentation issue forms now ask for one problem and AI assistance disclosure. The PR template asks for one focused change, base/fix regression evidence for bugs, checks actually run, compatibility impact, and AI assistance disclosure. Automated dependency updates and administrative metadata changes retain the owning-issue exception.

Compatibility

• No public compatibility impact
• Compatibility impact is documented in the owning issue and migration guidance

Verification

• Bug regression fails on the base and passes with the fix, or this is not a bug fix: not a bug fix
• Ruff lint and format checks pass
• ty passes
• Test suite passes: 2,261 passed, 14 skipped; 87.43% coverage
• Package build passes when applicable: not applicable to contributor text and templates
• Strict MkDocs build passes
• Ecosystem qualification passes: GitHub CI is pending

Commands run on commit 0c71a180d55a61dcdab66462396fb2ccd06b3898:

• uv sync --dev --extra docs --locked
• uv run ruff check src/ tests/ validation/
• uv run ruff format --check src/ tests/ validation/
• uv run ty check
• uv run pytest tests/ -q
• uv run mkdocs build --strict
• uv run python validation/check_documentation_links.py --site site: 8,769 rendered content links and 33 external destinations checked
• pre-commit run --all-files
• Parsed the three issue forms as YAML, checked unique field IDs and local contributor links and anchors, and confirmed the shared policy URL returned HTTP 200.

AI assistance

Codex drafted the guide, form and PR-template text, and this PR description. It checked the draft against the published ecosystem policy and the existing Backtest files, then ran the checks listed above. Human maintainer review is pending.

Documentation and release

This is contributor guidance. No package release is needed. Verify the root guide and README link on public main after merge.

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