ci(dogfood-gate): bump a2ml-validate-action to 6bff6ec (#176)

## Summary

Bump the SHA pin on \`hyperpolymath/a2ml-validate-action\` from
\`59145c7d\` to \`6bff6ec\` (HEAD of main) to pick up
[a2ml-validate-action#26](https://github.com/hyperpolymath/a2ml-validate-action/pull/26).

## Why

Dogfood Gate has been red on \`main\` for 5+ consecutive runs (since
2026-05-31 22:03Z), and the failure surfaced on every PR opened against
the repo as a \"Validate A2ML manifests\" error. Root cause:
\`audits/assail-classifications.a2ml\` uses s-expression form for its
identity fields —

\`\`\`a2ml
(assail-classifications
  (metadata
    (version \"1.1.0\")
    (project \"boj-server\")))
\`\`\`

— but the validator's regex only matched TOML form (\`name = \"...\"\`).
PR a2ml-validate-action#26 added the s-expression branches; bumping the
SHA here gets boj-server back to green.

## Test plan

- [ ] Dogfood Gate runs against this PR and reports \`Errors: 0\` on the
Validate A2ML step.
- [ ] Post-merge, \`Dogfood Gate\` workflow run on \`main\` passes (was:
failure since 2026-05-31).
- [ ] Spot-check the next main commit's Dogfood Gate run for green.

## Refs

- a2ml-validate-action#26 (the upstream regex fix, merged 11:36Z)
- boj-server#155 (the PR that added the s-expression entries)

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

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/dogfood-gate.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: Validate A2ML manifests
         if: steps.detect.outputs.count > 0
-        uses: hyperpolymath/a2ml-validate-action@59145c7d1039fa3059b3ecacdb50ee23d7505898 # main
+        uses: hyperpolymath/a2ml-validate-action@6bff6ec134fc977e86d25166a5c522ddea5c1e78 # main (post #26: s-expression identity recognition)
         with:
           path: '.'
           strict: 'false'