feat: Add comment-on-duplicates script for safer duplicate handling

Replace `gh issue comment:*` permission with a constrained script that:
- Only accepts validated issue numbers
- Enforces max 3 duplicates
- Uses a fixed comment format
- Prevents arbitrary comment content injection

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.claude/commands/dedupe.md

@@ -1,5 +1,5 @@
 ---
-allowed-tools: Bash(gh issue view:*), Bash(gh search:*), Bash(gh issue list:*), Bash(gh issue comment:*)
+allowed-tools: Bash(gh issue view:*), Bash(gh search:*), Bash(gh issue list:*), Bash(./scripts/comment-on-duplicates.sh:*)
 description: Find duplicate GitHub issues
 ---
 
@@ -11,28 +11,13 @@ To do this, follow these steps precisely:
 2. Use an agent to view a Github issue, and ask the agent to return a summary of the issue
 3. Then, launch 5 parallel agents to search Github for duplicates of this issue, using diverse keywords and search approaches, using the summary from #1
 4. Next, feed the results from #1 and #2 into another agent, so that it can filter out false positives, that are likely not actually duplicates of the original issue. If there are no duplicates remaining, do not proceed.
-5. Finally, comment back on the issue with a list of up to three duplicate issues (or zero, if there are no likely duplicates)
+5. Finally, use the comment script to post duplicates:
+   ```
+   ./scripts/comment-on-duplicates.sh --base-issue <issue-number> --potential-duplicates <dup1> <dup2> <dup3>
+   ```
 
 Notes (be sure to tell this to your agents, too):
 
 - Use `gh` to interact with Github, rather than web fetch
-- Do not use other tools, beyond `gh` (eg. don't use other MCP servers, file edit, etc.)
+- Do not use other tools, beyond `gh` and the comment script (eg. don't use other MCP servers, file edit, etc.)
 - Make a todo list first
-- For your comment, follow the following format precisely (assuming for this example that you found 3 suspected duplicates):
-
----
-
-Found 3 possible duplicate issues:
-
-1. <link to issue>
-2. <link to issue>
-3. <link to issue>
-
-This issue will be automatically closed as a duplicate in 3 days.
-
-- If your issue is a duplicate, please close it and 👍 the existing issue instead
-- To prevent auto-closure, add a comment or 👎 this comment
-
-🤖 Generated with [Claude Code](https://claude.ai/code)
-
----

scripts/comment-on-duplicates.sh

@@ -0,0 +1,86 @@
+#!/usr/bin/env bash
+#
+# Comments on a GitHub issue with a list of potential duplicates.
+# Usage: ./comment-on-duplicates.sh --base-issue 123 --potential-duplicates 456 789 101
+#
+
+set -euo pipefail
+
+REPO="anthropics/claude-code"
+BASE_ISSUE=""
+DUPLICATES=()
+
+# Parse arguments
+while [[ $# -gt 0 ]]; do
+  case $1 in
+    --base-issue)
+      BASE_ISSUE="$2"
+      shift 2
+      ;;
+    --potential-duplicates)
+      shift
+      while [[ $# -gt 0 && ! "$1" =~ ^-- ]]; do
+        DUPLICATES+=("$1")
+        shift
+      done
+      ;;
+    *)
+      echo "Unknown option: $1" >&2
+      exit 1
+      ;;
+  esac
+done
+
+# Validate base issue
+if [[ -z "$BASE_ISSUE" ]]; then
+  echo "Error: --base-issue is required" >&2
+  exit 1
+fi
+
+if ! [[ "$BASE_ISSUE" =~ ^[0-9]+$ ]]; then
+  echo "Error: --base-issue must be a number, got: $BASE_ISSUE" >&2
+  exit 1
+fi
+
+# Validate duplicates
+if [[ ${#DUPLICATES[@]} -eq 0 ]]; then
+  echo "Error: --potential-duplicates requires at least one issue number" >&2
+  exit 1
+fi
+
+if [[ ${#DUPLICATES[@]} -gt 3 ]]; then
+  echo "Error: --potential-duplicates accepts at most 3 issues" >&2
+  exit 1
+fi
+
+for dup in "${DUPLICATES[@]}"; do
+  if ! [[ "$dup" =~ ^[0-9]+$ ]]; then
+    echo "Error: duplicate issue must be a number, got: $dup" >&2
+    exit 1
+  fi
+done
+
+# Build comment body
+COUNT=${#DUPLICATES[@]}
+if [[ $COUNT -eq 1 ]]; then
+  HEADER="Found 1 possible duplicate issue:"
+else
+  HEADER="Found $COUNT possible duplicate issues:"
+fi
+
+BODY="$HEADER"$'\n\n'
+INDEX=1
+for dup in "${DUPLICATES[@]}"; do
+  BODY+="$INDEX. https://github.com/$REPO/issues/$dup"$'\n'
+  ((INDEX++))
+done
+
+BODY+=$'\n'"This issue will be automatically closed as a duplicate in 3 days."$'\n\n'
+BODY+="- If your issue is a duplicate, please close it and 👍 the existing issue instead"$'\n'
+BODY+="- To prevent auto-closure, add a comment or 👎 this comment"$'\n\n'
+BODY+="🤖 Generated with [Claude Code](https://claude.ai/code)"
+
+# Post the comment
+gh issue comment "$BASE_ISSUE" --repo "$REPO" --body "$BODY"
+
+echo "Posted duplicate comment on issue #$BASE_ISSUE"