feat(api): GitHub REST compat shim at /api/v3 — top 20 endpoint groups #5514

Scorecard move #5 of the greenlit block (agent-built, coordinator-rebased onto main)

Scripts and octokit-based tooling written against api.github.com now work against Gluecron by changing only the base URL (https://gluecron.com/api/v3 — the GitHub Enterprise convention) and the token. Authorization: token <PAT> and Bearer <PAT> both work.

All 20 endpoint groups: user/users, repo lists + repo object, branches/tags/commits (list + single with files/stats), contents GET and PUT (create/update with optimistic-sha 409), git refs, issues (list/get/create/PATCH close-reopen-edit), issue comments, pulls (list/get/create/PATCH), merge — routed through the shared performGatedMerge chain (200 {merged:true} / GitHub's 405 on refusal; never an inlined merge path), labels, rate_limit. GitHub-shaped errors ({message, documentation_url}), X-GitHub-Media-Type, real X-RateLimit-* from the shared limiter, Link-header pagination.

Documented shape compromises in the shapes-file header (UUID ids not integers; issues/PRs number from separate sequences; PR-list head.sha null to avoid a git spawn per row; /rate_limit an honest stub on GitHub's axis).

Native API (/api/v2) unchanged — this is a translation layer only. 48/48 tests; typecheck clean on current main.

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