feat: support YAML format for flaglint.config.json (.flaglintrc.yml)

[Krishan27]

Summary

The config loader in src/config.ts currently only reads flaglint.config.json. Many teams prefer YAML for config files because it supports comments and is less noisy for arrays and nested objects. Supporting .flaglintrc.yml / .flaglintrc.yaml as an alternative to flaglint.config.json lowers the friction for adopting FlagLint in projects that already use YAML-based tooling.

Proposed lookup order

When no --config path is passed explicitly, the loader tries in order:

1. flaglint.config.json (existing behaviour — unchanged)
2. .flaglintrc.json
3. .flaglintrc.yml
4. .flaglintrc.yaml

The first file found wins. If none is found, defaults apply (also existing behaviour).

Acceptance criteria

• .flaglintrc.yml and .flaglintrc.yaml are parsed and validated through the same Zod schema as JSON config
• An explicit --config path/to/custom.yml also works for YAML files
• A YAML config example is added to the docs config reference page
• Tests cover: JSON config (existing), YAML config (new), explicit --config with YAML path
• Error messages for invalid YAML config are clear (include the file path and the parse error)

Implementation notes

• Use the js-yaml package (already extremely common; tiny footprint) or Node's --experimental-strip-types to avoid a new runtime dependency — confirm which fits the project's dep philosophy
• Alternatively, a minimal YAML parser subset (just the types FlagLint uses: strings, numbers, booleans, arrays, objects) can be hand-rolled in ~50 lines to keep the dependency count at zero
• The Zod validation layer does not change — YAML is parsed to a plain object first, then validated identically to JSON

Files likely touched

• src/config.ts — extend loader to probe YAML filenames and parse YAML
• src/config/tests/config.test.ts — add YAML config test cases
• docs-src/ — update config reference page
• package.json — add js-yaml if that route is chosen (needs ADR comment if a new runtime dep)