docs: add engine.driver to frontmatter reference

[Copilot]

The frontmatter reference omitted engine.driver even though the field is schema-defined and compiler-validated. This created an inconsistency between the primary frontmatter reference and the engine-specific docs.

• What changed

  • Added a new Engine Driver (\engine.driver:`)entry todocs/src/content/docs/reference/frontmatter.md`.
  • Documented purpose and behavior at reference level, including Copilot SDK-mode implication.
  • Linked to the detailed engine docs section for constraints and supported formats.

• Reference example

  engine:
    id: copilot
    driver: .github/drivers/custom-copilot-driver.js

[github-actions]

Hey @copilot 👋 — thanks for picking up the engine.driver documentation gap! Filling in the missing frontmatter.md reference entry is a well-scoped, high-value fix for users who rely on it as their primary lookup.

A couple of things to address before this is ready for review:

• No changes yet — the PR is still a draft with 0 lines changed. Once the doc edit lands, make sure the description is updated from the WIP placeholder to a clear summary of what was added and why.
• Tests / validation — for pure doc PRs there may be no automated test requirement, but if the project has a doc-linting or link-checking step, confirm it passes.
• Description — replace the boilerplate opening with a concise summary: what field was added, where, and a one-liner on why it was missing.

If you'd like a hand finishing up, here's a ready-to-use prompt:

In docs/src/content/docs/reference/frontmatter.md, add a documented entry for the `engine.driver` field.

Steps:
1. Locate the `engine:` section in frontmatter.md.
2. Add a `driver:` sub-entry that matches the style of surrounding entries, describing the accepted values and their effect (refer to engines.md and main_workflow_schema.json for the allowed driver values).
3. Update the PR body to replace the WIP placeholder with a concise summary of the change.

Generated by ✅ Contribution Check · 112.9 AIC · ⌖ 10.7 AIC · ⊞ 6.2K · ◷

[Copilot]

Pull request overview

Adds the missing engine.driver frontmatter reference.

Changes:

• Documents driver overrides and Copilot SDK behavior.
• Adds a Copilot driver example and detailed-doc link.

Show a summary per file

File	Description
docs/src/content/docs/reference/frontmatter.md	Adds the engine.driver reference entry.

Review details

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• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Medium