feat: add counterfact.yaml config file support with --config option

[Copilot]

Summary

CLI options were becoming unwieldy to pass on every invocation. This adds a counterfact.yaml config file as an alternative — any CLI option can now be set there, with command-line flags always winning on conflict.

# counterfact.yaml
spec: ./openapi.yaml
port: 8080
serve: true
watch: true
proxy-url: https://api.example.com
prefix: /api/v1

Use --config <path> to point at a non-default location.

Original Prompt

The options in the command line are getting unwieldy. It's time to support a counterfact.yaml file as an alternative way to set configuration.

Follow common conventions, e.g. --config to set an alternate config file path; when there's a conflict, command line settings override config file settings

Manual acceptance tests

• Running with a counterfact.yaml in the current directory starts the server using config values (e.g. port: 8080 overrides the default 3100)
• An explicit CLI flag (e.g. --port 9000) overrides the same key set in counterfact.yaml
• --config ./path/to/other.yaml loads options from the specified file instead of counterfact.yaml
• --config ./nonexistent.yaml exits with a clear "Config file not found" error
• No counterfact.yaml present and no --config flag → startup proceeds normally with CLI/default values
• Kebab-case keys in the YAML (e.g. proxy-url, always-fake-optionals) are recognised correctly

Tasks

• Added src/util/load-config-file.ts: reads and parses YAML, normalises kebab-case keys to camelCase, returns {} when default file is absent, throws when a required file is missing
• Updated bin/counterfact.js: imports loadConfigFile, adds --config <path> Commander option, merges file config into options before building Config using program.getOptionValueSource() to detect explicit CLI flags
• Added 9 unit tests in test/util/load-config-file.test.ts using usingTemporaryFiles() per project convention
• Updated README.md and bin/README.md with the new option and example config
• Updated docs/usage.md: added --config to the CLI reference block and a new "Using a counterfact.yaml config file" <details> section covering the example YAML, kebab-case key normalisation, custom config path, and missing-file behaviour
• Added minor changeset

[pmcelhaney]

Use usingTemporaryFiles() for the test.

[Copilot]

Use usingTemporaryFiles() for the test.

Done in 4f14c8f — replaced the manual tmpdir/mkdir/writeFile helper with usingTemporaryFiles() throughout the test file.

[pmcelhaney]

Add documentation in usage.md

[Copilot]

Add documentation in usage.md

Done in 746b53e — added --config to the CLI reference block and a new "Using a counterfact.yaml config file" <details> section in docs/usage.md, covering the example YAML, kebab-case key normalisation, custom config path, and error/no-file behaviour.