feat: AgentKit — FastAPI-style ergonomics layer (0.4.0)

[ebarti]

What

A FastAPI-flavored ergonomics layer over the existing runtime API — zero new dependencies, everything additive on the 0.3.0 surface.

from dataclasses import dataclass
from agent_runtime_kit import AgentKit

@dataclass
class RepoSummary:
    name: str
    languages: list[str]

async with AgentKit() as kit:
    result = await kit.run(
        "claude",                     # alias; "claude-agent-sdk" works too
        goal="Summarize this repository",
        permissions="strict",         # literal -> real enum member
        output_type=RepoSummary,      # type -> wire schema -> typed .parsed
    )
    print(result.parsed.languages if result.parsed else result.error)

Commits (each green: pytest, mypy --strict, ruff)

1. PermissionProfile literal coercion — mode="strict" now coerces to the real enum member in __post_init__. Previously the bare string silently matched none of the adapters' is-identity checks and the run proceeded at DEFAULT posture; unknown values now raise with the valid vocabulary.
2. _schema.py + OutputTypeError — stdlib bridge between Python types and JSON schema: dataclasses, TypedDict, Literal, str-Enums, X | None, list[X], dict[str, X]. Fail-closed on anything else; strict parsing with $-path breadcrumbs; Pydantic v2 models used via their own model_json_schema/model_validate through a structural check (no import, no dependency).
3. AgentKit + ParsedResult[T] — kwargs-native run() assembling the frozen AgentTask (prebuilt task= passes through), KIND_ALIASES for the built-in kinds, per-kind runtime cache closed by aclose()/async with, and output_type= returning ParsedResult[T] (a runtime-identical AgentResult subclass; AgentResult stays non-generic so downstream disallow_any_generics users are unaffected). Non-conforming payloads yield finish_reason="failed" — the adapters' own structured-output convention — never an exception.
4. @kit.on(...) + @kit.runtime(...) — sync/async event handlers tee'd alongside the task's own sink (per-handler exception swallowing, the safe_emit contract), and decorator registration for third-party kinds.
5. Docs + 0.4.0 — quickstart/README lead with the hub, api-stability pins the posture (hub is sugar, aliases bounded, output_type subset documented and fail-closed).

Test delta

232 → 258 tests (test_schema.py, test_kit.py, coercion tests in test_core.py). No existing test, adapter, or protocol changed.