Restrict @claude bot to mcp org only, fix fork behavior (#819)

Written by Claude Code, reviewed by me.

## Summary

Fixes `@claude` to work on external fork PRs by:
1. Restricting triggers to modelcontextprotocol org members only
2. Properly checking out fork PR code via `refs/pull/N/head`

## Background

The `@claude` GitHub app was failing on external forks
([example](https://github.com/modelcontextprotocol/registry/actions/runs/19938435814/job/57169634431))
because the action tried to fetch branches by name, which doesn't work
for forks.

## Changes

### Org membership check
- Fetches the member list from
[`modelcontextprotocol/access`](https://github.com/modelcontextprotocol/access)
repo's `users.ts`
- Only org members can trigger `@claude` (prevents strangers from using
it)
- Uses `github.triggering_actor` so the person commenting must be an org
member

### Fork PR checkout fix
- Detects when a comment is on a fork PR
- Uses `refs/pull/{number}/head` to checkout fork code (instead of
branch name)
- This works because GitHub exposes all PRs via special refs on the base
repo

## How it works

1. Org member comments `@claude` on a fork PR
2. Workflow checks if commenter is in the `modelcontextprotocol/access`
member list
3. Workflow detects it's a fork PR and checks out via `refs/pull/N/head`
4. Claude runs with access to the fork's code

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/claude.yml

@@ -25,10 +25,43 @@ jobs:
       id-token: write
       actions: read
     steps:
+      # Only allow modelcontextprotocol org members to trigger @claude
+      # This enables @claude to work on external fork PRs when triggered by org members
+      # Members list is fetched from modelcontextprotocol/access repo
+      - name: Check if org member
+        run: |
+          ACTOR="${{ github.triggering_actor }}"
+          USERS_URL="https://raw.githubusercontent.com/modelcontextprotocol/access/main/src/config/users.ts"
+
+          # Fetch users.ts and extract GitHub usernames
+          MEMBERS=$(curl -fsSL "$USERS_URL" | grep -oE 'github:\s*"[^"]+"' | sed 's/github:\s*"//;s/"$//')
+
+          if echo "$MEMBERS" | grep -qxF "$ACTOR"; then
+            echo "User $ACTOR is a member of modelcontextprotocol org"
+          else
+            echo "::error::User $ACTOR is not a member of the modelcontextprotocol org. Only org members can trigger @claude."
+            exit 1
+          fi
+
+      # For PR comments, get PR details to checkout the correct branch (including forks)
+      - name: Get PR details
+        id: pr
+        if: github.event.issue.pull_request
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          PR_DATA=$(gh api ${{ github.event.issue.pull_request.url }})
+          echo "number=$(echo "$PR_DATA" | jq -r '.number')" >> $GITHUB_OUTPUT
+          echo "head_ref=$(echo "$PR_DATA" | jq -r '.head.ref')" >> $GITHUB_OUTPUT
+          echo "head_repo=$(echo "$PR_DATA" | jq -r '.head.repo.full_name')" >> $GITHUB_OUTPUT
+          echo "is_fork=$(echo "$PR_DATA" | jq -r '.head.repo.fork')" >> $GITHUB_OUTPUT
+
       - name: Checkout repository
         uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd
         with:
           fetch-depth: 1
+          # For fork PRs, checkout via PR ref; otherwise use the branch directly
+          ref: ${{ steps.pr.outputs.is_fork == 'true' && format('refs/pull/{0}/head', steps.pr.outputs.number) || steps.pr.outputs.head_ref || github.ref }}
 
       - name: Run Claude Code
         id: claude