docs: introduce frontmatter reference on first Quick Start path

[Copilot]

Quick Start first mentioned frontmatter in optional Step 4, so users skipping customization could hit deeper docs without context. This update introduces the term earlier on the default path and links directly to the frontmatter reference.

• Quick Start content update

  • Added an inline definition + link to frontmatter in the Step 2 flow (before optional customization), in the lock-file explanation block.
  • File: docs/src/content/docs/setup/quick-start.mdx

• Doc regression coverage

  • Added a focused Playwright spec to assert frontmatter appears before “Step 4 - Customize your workflow”.
  • File: docs/tests/quick-start-frontmatter.spec.ts

Workflow `.md` files start with [frontmatter](/gh-aw/reference/frontmatter/) — the configuration block between the `---` markers at the top of the file.

[pelikhan]

@copilot revert this file

[Copilot]

Done in b7b19a1.