Rename CONTRIBUTORS.md → CONTRIBUTING.md so GitHub surfaces the contributing policy

[cliffhall]

Sub-task of #1821. Blocked on #1866 merging — see below.

Why

GitHub's contributing-guidelines affordance — the banner shown when someone clicks "New issue" or opens a pull request — keys on the exact filename CONTRIBUTING.md. v2 named the file CONTRIBUTORS.md, so that prompt is dark precisely where the "don't open a PR, open an issue" message needs to land. The file's own heading already reads "Contributing to MCP Inspector", so no content change is needed — only the name and the references to it.

This also removes the need for the redirect-stub alternative considered in #1870, which would have left two files to keep in sync.

Safe against the merge model

main has never touched CONTRIBUTORS.md since the merge base (git log 4d30d1cd..origin/main -- CONTRIBUTORS.md is empty), and it never will — it's release-only. A rename conflicts only when the other side also modifies the file, so git will record a clean rename. Neither branch has a CONTRIBUTING.md today (v1/main has one, but v1 never merges into either branch).

Use git mv so history follows the file.

⚠️ Ordering

• Must wait for docs: post-swap branch model, PR policy, milestone rule, and Copilot review instructions #1866. That PR modifies CONTRIBUTORS.md (the ## Who opens pull requests section, the version table, the TL;DR line). Renaming first gives it a rename/modify conflict — the one genuinely annoying case.
• docs: unpin the branch-pinned CONTRIBUTORS.md links in the PR template #1879 already handles the PR-template links. It makes both .github/pull_request_template.md links relative, so they follow the rename automatically. Don't re-fix them here.

The work

1. git mv CONTRIBUTORS.md CONTRIBUTING.md
2. Update the reference in AGENTS.md (and re-grep — docs: post-swap branch model, PR policy, milestone rule, and Copilot review instructions #1866 adds links to it, so there may be more than one by the time this runs).
3. Check nothing else references the old name: git grep -n CONTRIBUTORS.

Note

External links to blob/*/CONTRIBUTORS.md — including in already-posted PR and issue comments — will 404 after this. That's acceptable and unavoidable; GitHub doesn't redirect deleted paths.

[cliffhall]

Closed by #1884, merged to v2/main. (Closing manually: Closes #N only auto-closes for PRs targeting the default branch, and v2 PRs target v2/main.)