ci: bring the main-source guard to main (bootstrap for #5119)

[cliffhall]

Part of #5119

Part of, not Closes: this PR targets main, the default branch, where a closing keyword would close #5119 at merge, before #5120 lands on v2/main.

Description

A one-off bootstrap. It brings the three files of the Source is v2/main guard from #5120 to main, byte-for-byte:

• .github/workflows/main-source.yml
• scripts/verify-main-source.mjs
• scripts/verify-main-source.test.mjs

The guard runs on pull_request_target, which loads the workflow and script from main. Until they are on main, the check cannot run on (or be required for) any PR into main. Docs changes stay on #5120 and reach main with the next milestone merge.

This is a deliberate one-time exception to "only milestone merges target main", made on the maintainer's decision, before the rule that enforces it exists. The guard itself would reject this PR, but it cannot run here (see above).

Server Details

None (repository-wide): CI and a root script.

Motivation and Context

#5119; the rationale and review are on #5120.

How Has This Been Tested?

• On this branch (main + the three files): npm ci && npm run validate:guards, exit 0. That covers every script test, including the 12 for the guard, plus the workflow guards (verify:action-pins, the workflow timeout and local-script rules). verify:dco -- --base origin/main --head HEAD: 1 commit, signed off.
• The three files are byte-identical to ci: guard main so a PR into it brings nothing not already on v2/main #5120's head (cmp).
• The next milestone merge stays pure. git merge-tree --write-tree <this head> <#5120 head> gives exactly ci: guard main so a PR into it brings nothing not already on v2/main #5120's tree (88bd7a36), so the release skill's tree-equality check still passes once both have merged. Keep it that way: if ci: guard main so a PR into it brings nothing not already on v2/main #5120 changes these files, this PR is updated to match before it merges.
• After merge: two throwaway PRs into main (one at origin/v2/main, expected to pass; one with a commit of its own, expected to fail) probe the live check before it is made required.

Breaking Changes

None.

Types of changes

• Bug fix
• New feature
• Breaking change
• Documentation update (CI tooling only)

Checklist

• I have read the MCP Protocol Documentation (not applicable: no protocol surface)
• My changes follow MCP security best practices: pull_request_target with no PR code checked out or run, read-only token
• I have updated the server's README accordingly (not applicable: no server changed)
• I have added a changeset (not applicable: no published package changes)
• I have tested this with an LLM client (not applicable: CI-only; see above)
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling (fails closed)
• I have documented all environment variables and configuration options (not applicable: none)

Merge with "Create a merge commit", the only method main now allows.

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: f39bc6b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[Copilot]

🟢 Approval recommended

The guard is fail-closed, securely avoids executing PR code, and matches #5120 byte-for-byte.

0 open findings

What changed in this PR

Bootstraps main with the trusted source guard needed to enforce pure merges from v2/main.

Changes:

• Adds a pull_request_target workflow for PRs into main.
• Adds fail-closed ancestry verification.
• Adds Git-history and workflow-security tests.

File	Description
.github/​workflows/​main-source.yml	Runs the read-only source guard.
scripts/​verify-main-source.mjs	Verifies the PR head exists on v2/main.
scripts/​verify-main-source.test.mjs	Tests guard behavior and workflow safety.

🧠 Review effort: Balanced

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.