docs: adopt shared focused-contribution policy - #75

Closes #74.

Adopt the published ML4T contribution policy through the local guide and existing issue/PR templates. Require coherent issues, focused changes, public-workflow failing-base/passing-fix evidence, independent numerical correctness evidence, AI disclosure and contributor accountability. Preserve dependency/administrative exceptions, Engineer checks, and private security reporting.

Reviewed head: 9e65cf9. Base: 4d8b181. The original guide/template work is retained. The nine-file diff also includes portable README URLs and their candidate-context CI validation, with behavioral regressions. No package API, dependency or supported-platform change.

The issue chooser's obsolete organization Discussions route returns HTTP 404. The canonical Engineer documentation returns HTTP 200 with the correct page identity using curl. Private reporting is unchanged and leads unauthenticated visitors to GitHub sign-in. The shared policy is published at ecosystem main 0cae766f4ec8911e4d0f91eb3360adfa0cd5ecce.

Package-context regression: rendering real wheel metadata from producer 9c3f892 with readme-renderer 46.0 resolves four license/contributor anchors to invalid PyPI-relative paths. The corrected wheel resolves all four to canonical GitHub targets. The currently published PyPI README is not changed by this PR alone.

CI regression: both hosted documentation jobs at 37311a1 fail because the new canonical main/CONTRIBUTING.md URL is not published until merge. The real checker reproduces that 404 locally. Ten permanent behavioral cases yield six failures and four passing external-URL controls on that producer. At the reviewed head all 28 release-candidate/policy tests pass. Exact canonical same-repository main-file URLs now resolve against candidate files, using the existing decoded-path containment and existence checks. Other hosts, owners, repositories and refs still require actual HTTP success. Missing candidate files and repository escapes fail even if the remote URL returns 200. No checks or workflow gates are disabled.

Local verification passed at the reviewed head: full suites on Python 3.12, 3.13 and 3.14 each have 4,276 passes, 57 intentional deselections and 17 known warnings; Ruff, formatting, ty, lock validation and all-file hooks; core and complete dependency audits with no known vulnerabilities; wheel/sdist build/import/twine; strict MkDocs; documented workflows from an isolated installed wheel; and the real README checker with external HTTP enabled. Previous template YAML/security-preservation checks remain pinned in the private evidence. All 30 applicable hosted checks passed, including all 15 protected contexts, all nine source cells and all nine installed-wheel cells with ten full suites per cell. Normal protected merge completed at 6b6d02c; its tree matches the reviewed/tested candidate exactly. Issue #74 closed through the merge. The canonical main contribution-guide GET now succeeds with the expected content. Final native main CI runs separately; no release or deployment has occurred.

The guide/templates and CI helper do not require a package release themselves. The README repair can ship with the separately recommended numerical-fix patch. This PR does not release or deploy anything.

AI assistance: Codex reviewed and reused the existing guide/template work, reproduced the route, packaged README and hosted validator failures, implemented the focused README/checker repairs and regression tests, ran the recorded checks, and prepared this description.