docs: stack object in pull_request webhooks

[Copilot]

pull_request webhook events now include a stack object for PRs that belong to a stack. Without documentation, teams can't easily discover how to use stack.base.ref to trigger CI on every PR in a stack targeting main — not just the bottom PR that directly targets main.

Changes

• New guide docs/src/content/docs/guides/ci-integration.md:

  • Documents the stack object shape in pull_request webhook payloads
  • Distinguishes pull_request.base.ref (direct PR base) from pull_request.stack.base.ref (ultimate stack target)
  • GitHub Actions if condition example covering both stacked and standalone PRs
  • JavaScript and Python webhook handler snippets with safe optional chaining for non-stacked PRs
  • Use cases for stack.base.sha (drift detection, targeted git log, CI caching)
  • Reference table mapping all four PR scenarios (standalone, bottom, mid-stack, top) to their field values

• docs/astro.config.mjs: Adds the new guide to the sidebar under Guides.

Key pattern documented

# GitHub Actions: run on any PR that ultimately targets main
if: |
  github.event.pull_request.base.ref == 'main' ||
  github.event.pull_request.stack.base.ref == 'main'

// Custom webhook handler — stack is absent for standalone PRs
const targetsMain =
  pr.base.ref === 'main' || pr.stack?.base?.ref === 'main';

[Copilot]

Pull request overview

Adds documentation for the pull_request.stack object surfaced in pull_request webhook payloads and links it from the docs sidebar, plus an FAQ entry showing how to access this data in GitHub Actions workflows.

Changes:

• Adds a new Webhooks reference page documenting the pull_request.stack.base.{ref,sha} fields.
• Extends the FAQ with a GitHub Actions snippet and a small reference table for github.event.pull_request.stack.
• Updates the docs sidebar to include the new Webhooks reference page.

Show a summary per file

File	Description
docs/src/content/docs/reference/webhooks.md	New reference page explaining the stack object in pull_request webhook payloads and how it relates to Actions.
docs/src/content/docs/faq.md	Adds an FAQ section showing how to read pull_request.stack in GitHub Actions expressions.
docs/astro.config.mjs	Adds “Webhooks” to the Reference sidebar.

Copilot's findings

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Comments suppressed due to low confidence (1)

docs/src/content/docs/reference/webhooks.md:45

• This is internally inconsistent: it says the stack object is only present when the PR belongs to a stack, but then says for standalone PRs the field is null. For JSON webhook payloads, either document that the property is omitted/absent for standalone PRs, or (if it really is present) show it explicitly as "stack": null in the example payload.

The `stack` object is **only present** when the pull request belongs to a stack. For standalone PRs, the field is null.

• Files reviewed: 3/3 changed files
• Comments generated: 3