Add agent skills for building integrations

[veewee]

Introduces .agents/skills/ — skills that teach a coding agent how to build an integration with this package. Aimed at consumers of the library, not at contributors.

.agents/skills/ is the cross-runtime location (Claude Code, Codex, Copilot CLI, Gemini CLI). .claude/skills is git-ignored so it can be a local symlink.

Six task-oriented skills

Each is usable on its own — asking for just a response model pulls only that skill.

Skill	Use when
generate-http-api-call	A whole endpoint: request + response + handler + tests. Orchestrates the rest.
configure-http-client	Client and transport setup: base URI, auth, logging, plugin order, preset choice. Once per API.
generate-http-request	A RequestInterface model: URI templates, parameters, BodyType.
generate-http-response	A decoded payload into a strictly typed value object.
generate-http-request-handler	The class that runs one call, and where error handling belongs.
test-http-integration	Mock client vs VCR cassettes, and what each layer should assert.

Reference files (loaded on demand, not up front) cover the plugin catalogue and priority levels, the full encoder/decoder matrix, psl/type usage, and VCR cassette handling.

Conventions they encode

1. One vertical slice per endpoint, grouped per endpoint on disk — not Model/ + Request/ + RequestHandler/ folders.
2. A handler interface per endpoint, so consumers mock one call instead of an API client.
3. Strictly typed response models, validated once at the boundary — never a stored raw array with ?? null accessors. psl/type is the recommended validator; symfony/serializer, valinor, webmozart/assert and plain PHP are documented alternatives, since we can't require psl of everyone.
4. Client and transport configured once per API, in a factory the tests reuse — so the tested plugin stack is the production one.
5. Error handling at the layer that owns it: client plugin → transport decorator → request handler, in that order of preference.
6. Handler tests run through the real transport against a recorded cassette; mock clients are for plugins, encoders and error paths.

Verification

• Every referenced API checked against this branch's source: ClientBuilder priority levels, FormatterBuilder, the *::createDecorator() formatters, all encoders and decoders, SerializerTransport::withOutputType(), PathNamingStrategy options, FilesystemRecorder regex filters, and psl/type's shape($elements, $allowUnknownFields).
• All 49 PHP snippets extracted and linted on PHP 8.4 — the three that don't parse standalone are deliberate catalogue fragments.
• All internal markdown links resolve.
• A baseline run without the skills produced exactly the drift they target: technical folder grouping, no handler interfaces, fromArray()/toArray() instead of parsed types, and handler tests mocking TransportInterface.

Examples use an imaginary "Crumbs Bakery" API (App\Infrastructure\Bakery, GET /orders/{orderId}) throughout, so snippets compose across skills.