feat(action): expose a keyed input so callers can attest keyless

forgeseal signs with its offline CA by default. A downstream assayward gate
discovers the SLSA provenance from the Sigstore bundle written beside it,
which a keyed run does not produce, so an action-based caller had no way to
generate gate-verifiable evidence. Add keyed (default true, unchanged
behavior) and pass --keyed=false through when set.

Author: sns45

action.yml

@@ -22,6 +22,10 @@ inputs:
   lockfile:
     description: 'Explicit path to lockfile (auto-detected if omitted)'
     required: false
+  keyed:
+    description: 'Sign with forgeseal offline CA (true, the default) instead of keyless Sigstore. Set false for Fulcio/Rekor keyless signing, which is what a downstream assayward gate needs in order to discover the provenance bundle.'
+    required: false
+    default: 'true'
   sign:
     description: 'Sign artifacts with Sigstore keyless signing'
     required: false
@@ -131,6 +135,9 @@ runs:
           if [ "${{ inputs.attest }}" = "false" ]; then
             ARGS+=("--attest=false")
           fi
+          if [ "${{ inputs.keyed }}" = "false" ]; then
+            ARGS+=("--keyed=false")
+          fi
           if [ "${{ inputs.vex-triage }}" = "true" ]; then
             ARGS+=("--vex-triage")
           fi