fix: describe tool schema fields and show one request payload

[horizonzzzz]

What and why

Fixes #6.

Models inspecting typesafe_evaluate consistently fail their first call because neither the JSON Schema nor the prompt guidelines showed what a valid payload looks like:

1. evaluationSchema in src/schema.ts had 0 property-level description fields on state, questions, type, instructions, and criteria. LLMs had to infer structural constraints from raw anyOf and patternProperties unions.
2. The canonical sample payload was only exposed to /typesafe test and the playground, while prompt guidelines contained usage policies but no syntax example.
3. Because the syntax reminder was only appended to validation error messages, a first-call failure was the only way for models to discover the expected object-map shape of questions.

This PR implements the suggested scope discussed in #6:

• Added description annotations to agent-authored schema fields: state, questions, type literals, instructions, criteria (per question type), and model.
• Exported a single-line request example in promptGuidelines reusing the existing module-level sample.
• Left array-to-map normalisation out of this PR to keep scope focused. Accepted input is unchanged.

Verification

• npm ci and npm run check pass: build (tsc -p tsconfig.build.json) and typecheck (tsc --noEmit) succeed cleanly.
• Changed behavior has offline tests, or this PR does not change behavior:
  • Added structural test in tests/schema.test.ts verifying all authored schema fields and question variants carry descriptions.
  • Added assertion in tests/extension.test.ts verifying promptGuidelines contains the single-line payload example without newlines.
  • All 82 cross-platform offline unit tests pass. (Note: On native Windows, the 8 pre-existing permission assertion failures related to POSIX 0o600 vs NTFS 0o666 occur identically before and after this change; CI runs on Linux and macOS where they pass).
• Documentation and changelog notes are updated where needed: accepted API and runtime behavior are unchanged.

Live testing: Not applicable. Transport and response validation are untouched.

Compatibility and release notes

• Fully backward-compatible. Runtime validation logic and accepted payloads are unchanged.
• Serialized schema length increased from 1968 to 2998 bytes (~260 tokens), adding self-documenting metadata without altering schema validation.
• No version bump: maintainer-owned release process respected.