docs: add protobuf codegen guide + scenario test example

[keugenek]

Summary

Per review feedback from @JCAlvar and @pkosiec: proto doesn't need a plugin — it's codegen + docs showing how to use generated types with existing plugins.

What changed

Removed: The proto plugin (packages/appkit/src/plugins/proto/) — no resources, no routes, no lifecycle. Not a plugin.

Added:

• docs/docs/guides/protobuf.md — guide on using buf/ts-proto codegen with AppKit's files and lakebase plugins
• examples/proto-catalog/ — scenario test demonstrating the pattern (Product Catalog app with Playwright tests)

The pattern

# 1. Define proto
echo 'message Customer { string id = 1; ... }' > proto/myapp/v1/models.proto

# 2. Generate types
npx buf generate proto/

# 3. Use with existing plugins (no proto plugin needed)
import { toJson, fromJson } from "@bufbuild/protobuf";
await app.files("data").upload(path, Buffer.from(JSON.stringify(toJson(schema, msg))));
const row = fromJson(schema, queryResult);

Scenario test (13/13 passing)

The examples/proto-catalog/ app demonstrates proto-style contracts without any proto plugin:

• 5 public cases: view all, filter, health check, snake_case fields
• 8 private cases: edge cases, 404, content-type, column headers

Test plan

• Proto plugin fully removed
• No @bufbuild/protobuf dependency in package.json
• Plugin exports reverted to pre-proto state
• Scenario test runs locally (13/13)
• Docs reframed as codegen guide
• pnpm build passes
• pnpm test passes