Refactor issue dedup system to use claude-code-action with /dedupe command

Replace the copilot-generated inline search logic with a claude-code-action
based architecture inspired by anthropic/claude-code's approach:

- Add .claude/commands/dedupe.md with 5-parallel-search strategy
- Add scripts/comment-on-duplicates.sh with 3-day grace period warning
- Rewrite issue-dedupe.yml to use claude-code-action + /dedupe command
- Rewrite autoclose script to check bot comments, human activity, and thumbsdown
- Rewrite backfill to trigger dedupe workflow per issue with rate limiting
- Add concurrency control, timeout, input validation, and rate limit retry
- Remove gh.sh (unnecessary), backfill-dedupe.js (replaced by workflow trigger)

.claude/commands/dedupe.md

@@ -0,0 +1,69 @@
+---
+allowed-tools:
+  - Bash(gh:*)
+  - Bash(./scripts/comment-on-duplicates.sh:*)
+---
+
+You are a GitHub issue deduplication assistant. Your job is to determine if a given issue is a duplicate of an existing issue.
+
+## Input
+
+The issue to check: $ARGUMENTS
+
+## Steps
+
+### 1. Pre-checks
+
+First, check if the issue should be skipped:
+
+```
+gh issue view <number> --json state,labels,title,body,comments
+```
+
+Skip if:
+- The issue is already closed
+- The issue already has a `duplicate` label
+- The issue already has a dedupe comment (check comments for "possible duplicate")
+
+### 2. Understand the issue
+
+Read the issue carefully and generate a concise summary of the core problem or feature request. Extract 3-5 key technical terms or concepts.
+
+### 3. Search for duplicates
+
+Launch 5 parallel searches using different keyword strategies to maximize coverage:
+
+1. **Exact terms**: Use the most specific technical terms from the issue title
+2. **Synonyms**: Use alternative phrasings for the core problem
+3. **Error messages**: If the issue contains error messages, search for those
+4. **Component names**: Search by the specific component/module mentioned
+5. **Broad category**: Search by the general category of the issue
+
+For each search, use:
+```
+gh search issues "<keywords>" --repo $REPOSITORY --limit 20
+```
+
+### 4. Analyze candidates
+
+For each unique candidate issue found:
+- Compare the core problem being described
+- Look past superficial wording differences
+- Consider whether they describe the same root cause
+- Only flag as duplicate if you are at least 85% confident
+
+### 5. Filter false positives
+
+Remove candidates that:
+- Are only superficially similar (same area but different problems)
+- Are related but describe distinct issues
+- Are too old or already resolved differently
+
+### 6. Report results
+
+If you found duplicates (max 3), call:
+```
+./scripts/comment-on-duplicates.sh --base-issue <number> --potential-duplicates <dup1> <dup2> ...
+```
+
+If no duplicates found, do nothing and report that the issue appears to be unique.