ci: bump a2ml-validate-action to 6bff6ec (s-expression form support) (#25)

Validator PR #26 (merged 2026-06-01) taught the action to recognise
both TOML `name = "x"` AND s-expression `(name "x")` identity
and version forms. The pre-fix SHA only matched TOML, so any
`.a2ml` file using s-expression dialect (idiomatic in campaign
reports and audit registries) failed Dogfood Gate with
"Missing required identity field" even when the field was
structurally present.

Real-world bite: hyperpolymath/panic-attack#94 hit this on
docs/campaigns/2026-05-26.a2ml on 2026-06-01.

This bump is purely a workflow SHA refresh; no behavioural change
for repos that don't carry s-expression manifests.

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
         with:
           path: '.'
           strict: 'false'