docs: clarify Bazaar contributor handbook

[TateLyman]

What

Extends the contributor handbook with the follow-up guidance discussed in #80: captured-response fixture contract shape, Bazaar discovery-variant triage for body-discovery vs MCP-discovery remediation copy, and no-payment-required test discipline for public 402/Bazaar proofs.

Linked Jira issue

Closes: N/A — maintainer-scoped docs follow-up from #80.

Change type

• Feature (new functionality)
• Fix (bug)
• Refactor (no behavior change)
• Docs
• Test
• CI
• Release
• Chore

Testing — per TESTING.md

• Unit tests added/updated (or N/A and explained below)
• Integration tests added/updated (or N/A and explained below)
• E2E test added (if user-facing change, or N/A and explained below)
• All existing tests still pass locally (pnpm test)
• pnpm typecheck clean
• pnpm lint clean
• Coverage did not drop by more than 1 point

N/A notes: docs-only PR; no runtime code or test assertions changed. I ran the captured-response integration test referenced by the new docs as a sanity check.

Manual verification output

$ pnpm lint:docs

> [email protected] lint:docs /private/tmp/x402trace-handbook
> echo 'TODO: wire up markdown lint in X402-2'

TODO: wire up markdown lint in X402-2

$ git diff --check

$ pnpm test tests/integration/bazaar-check-captured-responses.test.ts

> [email protected] test /private/tmp/x402trace-handbook
> vitest run tests/integration/bazaar-check-captured-responses.test.ts

 RUN  v3.2.4 /private/tmp/x402trace-handbook

 ✓ tests/integration/bazaar-check-captured-responses.test.ts (12 tests) 6ms

 Test Files  1 passed (1)
      Tests  12 passed (12)

Docs updated

• README — if user-facing
• SPEC.md — if scope changed
• ARCHITECTURE.md — if architecture changed
• CLAUDE.md — if hard rules or structure changed
• CHANGELOG.md — entry under [Unreleased]
• N/A — internal change with no doc impact

Risks / open questions

Docs-only; no runtime behavior, JSON shape, CI behavior, or paid-call path changed. I intentionally kept ADR/cost-discipline material out of scope per the maintainer guidance in #80.

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