feat: add Protobuf and gRPC API review skill 🤖🤖🤖

[Mnilax]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have reviewed the guidance for submissions involving paid services; this skill requires none.
• My contribution adds a new skill in the correct directory and follows the naming convention.
• The content is clearly structured and follows the Agent Skills format.
• I tested spec compliance and bundled-reference resolution with Vally.
• I ran the repository validation and build scripts and verified the generated skill index.
• I am targeting the main branch.

Description

Adds protobuf-grpc-api-review, an Agent Skill for reviewing Protocol Buffer and gRPC contract changes.

The skill helps an agent:

• separate binary-wire, ProtoJSON/text, generated-source, and behavioral compatibility
• trace old/new reader and writer combinations through rollout and rollback
• identify unsafe tag, type, cardinality, enum, presence, and oneof changes
• review deadlines, cancellation, retry/idempotency, status codes, streaming, and resource boundaries
• produce evidence-backed findings, a compatibility matrix, and a staged migration plan

Two progressively loaded references keep the main workflow compact while preserving detailed Protobuf and gRPC guidance. The rules are grounded in the official protobuf.dev and grpc.io documentation linked from those references.

This fills a current gap: the repository has no Protobuf- or gRPC-specific review skill, and no open PR or issue proposes one.

Type of Contribution

• New skill file.

Validation

• node eng/validate-skills.mjs — all 419 skills valid
• Vally lint — 2/2 checks passed (spec-compliance, valid-refs)
• node eng/update-readme.mjs — generated docs/README.skills.md
• node eng/generate-marketplace.mjs — completed successfully
• git diff --check — clean

Additional Notes

No paid services, credentials, scripts, or runtime dependencies are introduced.

By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[github-actions]

🔒 PR Risk Scan Results

Scanned 3 changed file(s).

Severity	Count
🔴 High	0
🟠 Medium	0
ℹ️ Info	0

✅ No matching risk patterns were detected in changed files.

This is an automated soft-gate report. Findings indicate review targets and do not block merge by themselves.

[github-actions]

🔍 Vally Lint Results

✅ All checks passed

Scope	Checked
Skills	1
Agents	0
Total	1

Severity	Count
❌ Errors	0
⚠️ Warnings	0
ℹ️ Advisories	0

Summary

Level	Finding
ℹ️	✅ protobuf-grpc-api-review (2/2 checks passed)
ℹ️	✓ [spec-compliance] All 1 skill(s) are spec-compliant.
ℹ️	✓ spec-compliance: All spec checks passed.
ℹ️	✓ [valid-refs] All file references across 1 skill(s) are valid.
ℹ️	✓ valid-refs: All file references resolve to existing files within the skill directory.
ℹ️	1 skill(s) linted, 1 passed

Full linter output

### Linting skills/protobuf-grpc-api-review
✅ protobuf-grpc-api-review (2/2 checks passed)
    ✓ [spec-compliance] All 1 skill(s) are spec-compliant.
        ✓ spec-compliance: All spec checks passed.
    ✓ [valid-refs] All file references across 1 skill(s) are valid.
        ✓ valid-refs: All file references resolve to existing files within the skill directory.

1 skill(s) linted, 1 passed