Adopt the shared contribution policy in Backtest guidance and templates #134

Problem

Backtest has issue forms and a pull-request template, but no local CONTRIBUTING.md entry point to the published ecosystem contribution policy. The forms and template do not yet ask contributors for one problem per issue, one focused PR, real regression evidence for bugs, or AI assistance disclosure and accountability. This leaves maintainers without the information needed to review fixes consistently.

Required outcome

Add a concise Backtest contribution guide linked to the published shared policy. Preserve the existing local issue fields and quality commands while updating the forms and PR template to request the shared requirements. Keep administrative metadata and automated dependency updates exempt from the owning-issue rule. Direct suspected vulnerabilities to the private reporting path.

Acceptance

• A contributor can find the local guide in the public repository, follow its link to the published shared policy, and identify Backtest's quality commands.
• Bug reports ask for one reproducible problem, affected version and environment, expected and observed behavior, and a complete reproduction.
• PRs ask for one focused change, a closing issue reference when applicable, base/fix regression revisions and results for bugs, compatibility impact, checks actually run, and AI assistance disclosure.
• Documentation, feature, and security intake preserve their existing routes.
• Repository quality checks and strict documentation build pass; rendered contributor links resolve.

Shared policy and setup remain owned by ml4t/ecosystem and ml4t-dev. This issue covers only Backtest's local adoption.

AI assistance

Codex drafted this issue from the published ecosystem policy and the inspected Backtest templates. It checked the stated scope against those files. Maintainer review of the proposed public changes is pending.