Run the Axiom parity check on pull requests (warn-only)

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,3 +25,6 @@ Description of the changes here.
 ## What this fixes and how it's fixed
 
 Description of how this fix works goes here. Link any issues this PR fixes.
+
+<!-- Required for any change to policy (programs, parameters, eligibility, bug fixes): say how the same provision is right in TheAxiomFoundation/rulespec-ca (PolicyEngine/.github CONTRIBUTING, "Mirror policy changes in Axiom"). Keep one option below (several claims can share the line, separated by `;`), or use axiom: needed if you are an external contributor who cannot complete the Axiom work; a maintainer will follow up. The Axiom parity check verifies the line and comments with suggestions. -->
+axiom: <legal id> encoded-correct (<companion test>) | <rulespec-ca PR> encoded | <rulespec-ca issue> queued | n/a: <category>: <reason>

.github/workflows/axiom-parity.yaml

@@ -0,0 +1,20 @@
+# Checks the PR description's axiom: line (PolicyEngine/.github CONTRIBUTING,
+# "Mirror policy changes in Axiom") and suggests one. Warn-only for now: it
+# reports what would fail in the job summary and never blocks merging.
+name: Axiom parity
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened, ready_for_review]
+
+permissions:
+  contents: read
+
+jobs:
+  axiom-parity:
+    uses: PolicyEngine/.github/.github/workflows/axiom-parity.yml@main
+    with:
+      mode: warn
+    permissions:
+      contents: read
+      pull-requests: write

changelog.d/axiom-parity-check.changed.md

@@ -0,0 +1 @@
+Run the Axiom parity check on pull requests (warn-only for now) and add the axiom line to the PR template.