feat(adapters): add Gitea and Forgejo REST adapters - #1707

Summary

• Add a shared forge-neutral REST adapter with bounded pagination, concurrency, request budgets, response sizes, and token-safe errors.
• Add private Gitea and Forgejo adapter profiles for comparisons, merged pull requests, changed files, contributors, refs, tags, and release operations.
• Implement PullRequestReader through /pulls/{number} and normalize number, title, labels, and base branch for check-pr.
• Add adapter contract, declaration, package-consumer, metadata, and boundary coverage.

fixes #1386

Compatibility

The adapters remain private and are not wired into the public package in this layer. Existing public API and GitHub Action behavior do not change, and the tracked root Action bundle remains unchanged.

Validation

• npm run ci
• npm run check:clean
• Adapter behavior, malformed response, declaration, package-consumer, and bundle-boundary checks passed.

Stack

Stacked on #1706. The GitLab adapter follows in #1708, and public multiforge wiring follows in #1709.

Part of #1696.

Review size

• Reviewable changes: +2,248 -9 across 13 files
• Generated artifacts: +20 -0 across 1 file

GitHub's headline total includes both rows. .gitattributes marks generated artifacts and collapses their diffs by default.