Require focused and accountable library contributions

[stefan-jansen]

Require one problem per issue and one focused PR, with a minimal reproduction, executed regression evidence, and contributor accountability for AI-assisted code and prose. Verified numerical defects remain eligible for correction; proposed changes to intended conventions require agreement on expected behavior and compatibility impact.

Update the shared contributing guide, issue forms, PR template, and issue/PR standard for the current ML4T and maintained Reloaded libraries. Shared maintenance automation prepares recommendations for maintainer approval.

Verification:

• Ruff lint and format checks passed.
• ty passed.
• 110 tests passed; coverage was 91.94%, above the required 90%.
• Strict MkDocs build, package build, and all pre-commit hooks passed.

AI assistance: Codex drafted these policy and template changes from the maintainer's requirements and ran the listed checks. No runtime code changed.

Closes #47

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