feat(clang-tidy): add cache pruning for full-repo runs

[xmfcx]

• Parent Issue: feat(clang-tidy): add clang-tidy-cache support for faster CI runs #402
• Add clang-tidy-cache-prune input that removes stale ctcache entries not accessed during the current run
• Uses a timestamp marker before analysis to identify entries that were not hit by ctcache
• Includes grouped diagnostic logging showing cache entries/size before analysis, after analysis, and after prune

Why

The ctcache directory grows over time as files are added, renamed, or deleted across commits. In practice the full-repo cache is only ~7 MB so this is not urgent, but it's cheap to keep tidy. This input is intended for main branch runs that analyze all files — PR runs that only touch a subset should not prune, since they wouldn't access entries for files outside their scope.

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• Builds on feat(clang-tidy): add ctcache support for faster CI runs #403

Test plan

• Monitor cache size over several main branch runs before enabling pruning
• Once growth is observed, enable clang-tidy-cache-prune: true on a main branch run and verify the ctcache prune log group shows correct stale entry removal
• Verify that cache entries accessed during the run survive pruning (compare entry counts before/after)