feat: add pydantic agentic engine with uv support and smoke test

[Copilot]

Adds pydantic as a ninth built-in engine backed by pydantic-ai[cli], installed via uv. Extends the BehaviorDefinedEngine infrastructure (previously npm-only) to support Python/uv tooling.

Engine infrastructure

• pkg/workflow/python.go — new uv install step generators mirroring nodejs.go (GenerateUvSetupStep, GenerateUvToolInstallSteps, BuildUvEngineInstallStepsWithAWF); uses the already-pinned astral-sh/setup-uv@v8.2.0
• pkg/workflow/behavior_defined_engine.go — replaced early-return-on-non-npm guard with a switch on PackageManager to dispatch to either npm or uv install paths

New engine

• pkg/workflow/data/engines/pydantic.md — declarative definition: package-manager: uv, secret-strategy: universal-llm-consumer, model env var PYDANTIC_AI_MODEL
• pkg/workflow/agentic_engine.go — registered "pydantic" alongside opencode/crush in the BehaviorDefinedEngine loop
• pkg/constants/engine_constants.go — PydanticEngine constant, appended to AgenticEngines, added EngineOptions entry

Smoke test & docs

• .github/workflows/smoke-pydantic.md — minimal smoke workflow exercising the engine end-to-end
• docs/src/content/docs/reference/engines.md — pydantic row added to all engine tables

[pelikhan]

@copilot as a shared agentic workflow, not in the go compiler/code

[Copilot]

@copilot as a shared agentic workflow, not in the go compiler/code

Restructured as a shared agentic workflow. The pydantic installation (uv setup + pydantic-ai[cli] install) now lives in .github/workflows/shared/pydantic-engine.md — consumers just add imports: [shared/pydantic-engine.md] and configure the engine command inline. The python.go uv infrastructure and behavior_defined_engine.go switch have been removed from the compiler. Only the minimal PydanticEngine constant remains in Go so engine: id: pydantic resolves correctly. Done in commits starting with the one that adds shared/pydantic-engine.md.