ci+docs: Add draft PR enforcement (#5867)

Add a GitHub Actions workflow that automatically converts non-draft PRs
to draft on open/reopen, and document the draft PR requirement in
CONTRIBUTING.md.

**Motivation:** Our [code submission
standard](https://develop.sentry.dev/sdk/getting-started/standards/code-submission/#pull-requests)
says PRs must start as drafts, but this wasn't enforced. Non-draft PRs
trigger review notifications prematurely and signal readiness when the
author may not be done. This workflow nudges contributors to open as
draft by automatically converting non-draft PRs and posting a comment
explaining the policy.

**What's included:**
- `.github/workflows/enforce-draft-pr.yml` — triggers on
`pull_request_target` (opened, reopened), uses the GraphQL
`convertPullRequestToDraft` mutation. Includes error handling and
comment deduplication on reopen.
- `CONTRIBUTING.md` — new "Pull Requests" section documenting the draft
requirement and linking to the code submission standard.

**Limitations:** GitHub has no pre-creation hook, so the initial CI run
and review notifications still fire before the conversion. The value is
in training contributors and ensuring draft state before a maintainer
picks up the PR.

No exemptions — applies to everyone (maintainers, internal, external
contributors).

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stephanie-anderson

.github/workflows/enforce-draft-pr.yml

@@ -0,0 +1,75 @@
+name: Enforce Draft PR
+
+on:
+  pull_request_target:
+    types: [opened, reopened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  enforce-draft:
+    name: Enforce Draft PR
+    runs-on: ubuntu-24.04
+    if: github.event.pull_request.draft == false
+    steps:
+      - name: Convert PR to draft
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
+        with:
+          script: |
+            const pullRequest = context.payload.pull_request;
+            const repo = context.repo;
+
+            // Convert to draft via GraphQL (REST API doesn't support this)
+            try {
+              await github.graphql(`
+                mutation($pullRequestId: ID!) {
+                  convertPullRequestToDraft(input: { pullRequestId: $pullRequestId }) {
+                    pullRequest {
+                      isDraft
+                    }
+                  }
+                }
+              `, {
+                pullRequestId: pullRequest.node_id
+              });
+            } catch (error) {
+              core.warning(`Failed to convert PR to draft: ${error.message}`);
+              return;
+            }
+
+            // Label the PR so maintainers can filter/track violations
+            await github.rest.issues.addLabels({
+              ...repo,
+              issue_number: pullRequest.number,
+              labels: ['converted-to-draft'],
+            });
+
+            // Check for existing bot comment to avoid duplicates on reopen
+            const comments = await github.rest.issues.listComments({
+              ...repo,
+              issue_number: pullRequest.number,
+            });
+            const botComment = comments.data.find(c =>
+              c.user.type === 'Bot' &&
+              c.body.includes('automatically converted to draft')
+            );
+            if (botComment) {
+              core.info('Bot comment already exists, skipping.');
+              return;
+            }
+
+            const contributingUrl = `https://github.com/${repo.owner}/${repo.repo}/blob/master/CONTRIBUTING.md`;
+
+            await github.rest.issues.createComment({
+              ...repo,
+              issue_number: pullRequest.number,
+              body: [
+                `This PR has been automatically converted to draft. All PRs must start as drafts per our [contributing guidelines](${contributingUrl}).`,
+                '',
+                '**Next steps:**',
+                '1. Ensure CI passes',
+                '2. Fill in the PR description completely',
+                '3. Mark as "Ready for review" when you\'re done'
+              ].join('\n')
+            });

CONTRIBUTING.md

@@ -25,6 +25,18 @@ We will review your pull request as soon as possible. Thank you for contributing
 
 You are welcome to use whatever tools you prefer for making a contribution. However, any changes you propose have to be reviewed and tested by you, a human, first, before you submit a pull request with them for the Sentry team to review. If we feel like that didn't happen, we will close the PR outright. For example, we won't review visibly AI-generated PRs from an agent instructed to look for and "fix" open issues in the repo.
 
+## Pull Requests
+
+All PRs must be created as **drafts**. Non-draft PRs will be automatically converted to draft. Mark your PR as "Ready for review" once:
+
+- CI passes
+- The PR description is complete (what, why, and links to relevant issues)
+- You've personally reviewed your own changes
+
+A PR should do one thing well. Don't mix functional changes with unrelated refactors or cleanup. Smaller, focused PRs are easier to review, reason about, and revert if needed.
+
+For the full set of PR standards, see the [code submission standard](https://develop.sentry.dev/sdk/getting-started/standards/code-submission/#pull-requests).
+
 ## Development Environment
 
 ### Set up Python