Add LLM guidance, scaffold generator, exchangerate.host example, and WebMock testing docs

[kalashnikovisme]

Motivation

• Make the repository LLM-friendly so agents and humans can reliably generate Purple::Client wrapper clients using a clear policy, examples, and mapping guidance.
• Provide a real-world example (exchangerate.host) that demonstrates the DSL features (params, optional/blank, Purple::Boolean, response transforms) and a runnable spec pattern.
• Provide an easy way to scaffold new wrapper skeletons to encourage consistent wrappers and reduce manual errors.
• Use RSpec + WebMock for wrapper testing guidance (explicitly avoid VCR) and do not add any RBS/signature expansion work per constraints.

Description

• Added an LLM-focused guide AGENTS.md that explains what a wrapper client is/is not, golden-path structure, naming conventions, modeling rules (params, body, response, optional/allow_blank, Purple::Boolean, :array_of), DO/DON'Ts for LLMs, and a copy-paste skeleton template.
• Added OpenAPI-to-DSL mapping docs/openapi_mapping.md and testing guidance docs/testing_wrappers.md that show how OpenAPI concepts map to the Purple DSL and how to test with RSpec + WebMock (worked example included).
• Added a real-world example under examples/real_world/exchangerate_host/ including client.rb (live, historical, convert, timeframe endpoints demonstrating required/optional params, nested response shapes, optional/allow_blank, Purple::Boolean, and a response transform) and README.md documenting how to run it.
• Added a scaffold CLI bin/purple-client which implements bin/purple-client scaffold PROVIDER_NAME [--force] to generate lib/<provider>/client.rb, lib/<provider>/version.rb, lib/<provider>/README.md, and spec/<provider>/client_spec.rb while refusing to overwrite existing files unless --force is passed; made the CLI executable and documented the command in README.md.
• Added WebMock to Gemfile and included a new wrapper spec spec/exchangerate_host/client_spec.rb using RSpec + WebMock to validate requests and responses.
• Minor clarification edits: clarified that params is used to build request bodies for POST/PUT/PATCH and left body(...) for response schemas; adjusted the exchangerate example timeframe rates shape to use { type: Hash, allow_blank: true } to avoid overly strict validation.
• File list (added/modified): AGENTS.md, docs/openapi_mapping.md, docs/testing_wrappers.md, examples/real_world/exchangerate_host/client.rb, examples/real_world/exchangerate_host/README.md, spec/exchangerate_host/client_spec.rb, bin/purple-client, Gemfile, and updates to README.md.

Testing

• Attempted bundle install to fetch test deps but it failed due to a network/Gem::HTTP 403 from rubygems.org in this environment, so gems could not be installed (failure: network/403).
• Attempted bundle exec rspec but it failed because bundler/executables were not available due to the failed install (failure: missing gems); nevertheless, RSpec spec files were added and follow the existing test style and WebMock stubbing pattern.
• Verified scaffolder file output and permissions by creating bin/purple-client and making it executable, and validated generated file contents via inspection; the CLI will refuse to overwrite files unless --force is given.
• Manual validations performed: lint-free file writes, README/docs links added, and example/spec source was exercised locally via static inspection; full automated test run requires successful bundle install in an environment with network access.

Run instructions

• Install dependencies and run tests: bundle install then bundle exec rspec (note: in CI/dev with network access bundle install should succeed and specs will run).
• Run exchangerate.host example (optional live calls): ruby -r "./examples/real_world/exchangerate_host/client" -e "puts ExchangerateHost::Client.live(base: 'USD', symbols: 'EUR').rates[:eur]".
• Scaffold a wrapper: bin/purple-client scaffold my_provider and re-run with --force to overwrite.

Notes

• Per requirements, no types/RBS expansion was added; testing guidance uses RSpec + WebMock (no VCR).

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