docs: Add existing-review checks to dispatch and PR review guides

Agents were recommending PRs for review without checking whether
reviews had already been posted, leading to wasted effort. Added
Step 0 to dispatch_guide.md and Section 3.2.1 to pr_review_guide.md
requiring agents to query the GitHub reviews API before proceeding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TimDettmers

agents/dispatch_guide.md

@@ -12,6 +12,27 @@ python3 agents/fetch_issues.py
 
 Read `agents/github_tools_guide.md` for the full reference on how to use the query tools.
 
+## Step 0: Check Existing Reviews on Open PRs
+
+Before looking at issues, check whether there are open PRs from external contributors that need review. But **do not assume a PR needs review just because it is open** — check whether a review has already been posted.
+
+```bash
+# List open PRs
+gh pr list --state open --limit 30
+
+# For each external contributor PR, check for existing reviews
+gh api repos/bitsandbytes-foundation/bitsandbytes/pulls/<NUMBER>/reviews \
+  --jq '.[] | "\(.user.login) | \(.state) | \(.submitted_at)"'
+```
+
+A PR only needs a new review if:
+
+- **No review exists at all** from a maintainer or agent
+- **The author has pushed new commits** since the last review (check commit dates vs review dates)
+- **The author has responded to review feedback** and the review needs a re-review
+
+If a review already exists and the author has not responded or pushed changes, the ball is in the author's court — skip that PR. Do not generate a prompt to re-review work that has already been reviewed.
+
 ## Step 1: Find Candidate Issues
 
 Start by getting the landscape of open issues:

agents/pr_review_guide.md

@@ -148,6 +148,25 @@ From the metadata, note:
 - **Existing reviews and comments**: Has anyone already reviewed? Are there unresolved
   threads?
 
+### 3.2.1 Stop If Already Reviewed
+
+Before proceeding with a full review, check whether a substantive review has already been posted — by you, another agent, or a maintainer:
+
+```bash
+gh api repos/bitsandbytes-foundation/bitsandbytes/pulls/<NUMBER>/reviews \
+  --jq '.[] | "\(.user.login) | \(.state) | \(.submitted_at) | body_len=\(.body | length)"'
+```
+
+**Do not duplicate an existing review.** If a review with substantive feedback (body length > 500 characters) already exists and the author has NOT pushed new commits or responded since that review, stop here — the PR does not need another review. The ball is in the author's court.
+
+A new review is warranted only if:
+
+- No substantive review exists yet
+- The author has pushed commits after the last review (compare commit dates with review `submitted_at`)
+- The author has responded to feedback and the reviewer requested re-review
+
+This check prevents wasted effort reviewing PRs that are already waiting on the author.
+
 ### 3.3 Size Calibration
 
 Use the PR size to calibrate your review depth: