fix: resolve $ref pointers in operation parameters

[lucianfialho]

Problem

Specs that declare parameters once in `components.parameters` and reference them via `$ref` from operations (Stripe, Slack, GitHub, Sympla, etc.) crash spec2cli with:

```
Error: Cannot add option '--undefined ' to command 'X' due to conflicting flag '--undefined'
```

The extractor was resolving `$ref` for schemas only — parameter `$ref` entries were treated as raw objects, leaving `name` and `in` undefined.

Fix

• New `refs.ts` module exporting `resolveSchema` and `resolveParameter` (deduplicates the pointer-walking logic both already needed)
• `extractParams` now calls `resolveParameter` before reading `name`/`in`, and skips entries that still lack these after resolution (defensive)
• Split `extractor.ts` into focused modules (`refs`, `params`, `naming`) so each file stays under the 150-line gate

Test plan

• All 123 existing tests pass
• Sympla spec (which uses `$ref: '#/components/parameters/TokenUser'`) now loads — verified end-to-end with `--dry-run`
• Petstore and other specs without parameter refs still work

🤖 Generated with Claude Code