M141-2 step 3 — per-event payload schemas

Payload schema — PreToolUse

What this PR adds

• Switched codegen YAML parser from inline to js-yaml — spec can now nest payload.required + payload.optional blocks
• Emitted PayloadMap interface + PayloadFor<E> type alias for typed-payload consumers
• Emitted PAYLOAD_SCHEMAS data constant in schemas.ts — keyed by event name, used by envelopeSchema() to nest payload properties + required arrays
• 43 new vitest cases in tests/payloads.test.ts — covers properties block presence, envelope-only fallback, field-level structural checks, and the invariant that required ⊆ properties for every event

What's still deferred (steps 2 + 4)

• Step 2: PyPI sibling orchestkit-hook-contract — same spec, Pydantic models, JSON Schema mirrors
• Step 4: Cross-language parity gate CI — fails when npm and PyPI emit diverge on any event's field set
• The 6 uncovered events (SessionEnd, Setup, InstructionsLoaded, WorktreeCreate, WorktreeRemove, ConfigChange) — their CC-level payload shapes aren't documented enough to lock down yet