Constrain dedupe candidates to canonical open issues

[vishnukannaujia]

Why

Duplicate-bot suggestions can create noisy/circular duplicate webs and can point to poor canonical targets.

This is described in:

• [Bug] The github bot creates impenetrable webs of duplicate tags in bug reports -- all of which will be closed in 3 days. #19267
• [BUG] Duplicate bot suggests closed issues as duplicates, leading to incorrect auto-closure #20282

What changed

Updated .claude/commands/dedupe.md with explicit required candidate constraints before posting duplicate suggestions:

• candidate must be open
• candidate must be older than base issue
• candidate must not be labeled duplicate
• candidate must not be the base issue itself
• if no candidates satisfy constraints, do not post duplicate comment

Also added canonical preference guidance to avoid unstable targets (e.g. stale/autoclose).

Impact

• Reduces duplicate-of-duplicate chains.
• Prevents suggesting non-canonical or low-signal targets.
• Improves reliability of duplicate comments used by downstream auto-close flow.

Related

• Addresses [Bug] The github bot creates impenetrable webs of duplicate tags in bug reports -- all of which will be closed in 3 days. #19267
• Addresses [BUG] Duplicate bot suggests closed issues as duplicates, leading to incorrect auto-closure #20282

[vishnukannaujia]

Requesting maintainer review for this dedupe change: @ddworken @bogini @ashwin-ant . Could one of you please take a look when possible?