feat: add skip field to policy group attachments

Add support for explicitly disabling specific policies within a policy group
by specifying their metadata names in a skip list. This allows users to
selectively exclude policies from evaluation without modifying the policy
group itself.

Changes:
- Add skip field to PolicyGroupAttachment protobuf message
- Implement getPolicyName() helper to extract policy names from attachments
- Filter skipped policies in both material and attestation evaluation paths
- Add validateSkipList() to warn about non-existent policy names
- Add comprehensive test coverage for skip functionality

Example usage:
```yaml
policyGroups:
  - ref: file://groups/sbom-quality.yaml
    with:
      bannedLicenses: AGPL-3.0
    skip:
      - sbom-present
      - license-check
```

Policies are matched by metadata.name. Unknown policy names in the skip list
generate warnings but allow execution to continue.

Closes #2557

Signed-off-by: Miguel Martinez <miguel@chainloop.dev>

Author: migmartri

app/cli/pkg/action/testdata/contract_v2.yaml

@@ -33,4 +33,8 @@ spec:
     attestation:
       - ref: file://attestation-policy.yaml
   policyGroups:
-    - ref: file://testdata/policy_group.yaml
+    - ref: file://testdata/policy_group.yaml
+      with:
+        user_name: "test-user"
+      skip:
+        - policy-to-skip

app/controlplane/api/gen/frontend/workflowcontract/v1/crafting_schema.ts

@@ -344,6 +344,8 @@ message PolicyGroupAttachment {
   string ref = 1 [(buf.validate.field).string.min_len = 1];
   // group arguments
   map<string, string> with = 2;
+  // policy names to skip (matched against metadata.name)
+  repeated string skip = 3;
 }
 
 // Represents a group or policies