Document one upstream pilot and fork-only feature development

[ratovarius]

Change

Use upstream googleworkspace#937 (credential preservation) as the only active upstream pilot. Close upstream PRs 930–936 by author request, with pointers to the maintained public fork. All implementations and branches remain available; further development proceeds through this fork's issues and PRs.

Update README, FORK.md, and CONTRIBUTING.md with the active pilot, closed-proposal statuses, and fork-only development policy. The pilot includes its own small Clippy prerequisite and requires no other PR to merge.

Validation

• Exactly the four intended documentation/changeset files changed; whitespace and proposal-status checks passed.
• The upstream credential-fix branch was freshly validated at d89eec8: 787 Rust tests, formatting, strict Clippy, and build passed with isolated synthetic configuration.
• Google's actual flow is documented separately in the pilot PR: CLA coverage and upstream maintainer approval for workflows are still pending. Local validation is not presented as completed upstream CI.
• This PR changes no application, test, dependency, or workflow code. Existing implementations remain in fork main.

Fixes #11