Add duplicate thumbnail validation

[benceruleanlu]

Summary

• add a lightweight pair-based checker for byte-identical and visually equivalent thumbnails across distinct templates
• resolve explicit image thumbnail paths and implicit template-N.webp assets while ignoring multiple assets owned by one template
• add a changed-files/base-ref CI gate so unrelated legacy duplicates do not block PRs
• document the local commands and require the check in AGENTS.md for thumbnail changes

Measurements

• current corpus: 597 templates and 767 effective image thumbnails
• audit runtime: 2.33 to 2.52 seconds locally, median 2.36 seconds
• existing debt: 3 cross-template pairs, consisting of 1 byte-identical and 2 visually equivalent pairs
• the MiniMax and Nano Banana regressions are both detected despite different byte hashes

Validation

• 6 focused tests passed
• Ruff lint and format checks passed
• Python compilation passed
• workflow YAML parsed successfully
• audit mode reports all 3 legacy pairs without failing
• base-ref mode ignores unrelated legacy pairs and fails with actionable template IDs and asset paths for a newly affected duplicate

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