docs: add github-app to Allowed Import Fields in imports reference

[Copilot]

github-app is a supported top-level import field (compiler merges it with main-workflow-wins precedence) but was absent from the imports reference, leaving users uncertain whether centralizing app credentials in a shared import was intentional or accidentally working.

Changes

• docs/src/content/docs/reference/imports.md
  • Added github-app: to the Allowed Import Fields list
  • Added github-app: row to the Field-Specific Merge Semantics table with the correct precedence rule: main workflow wins; first imported value fills in if main omits it

Example shared import for the side-repo-ops pattern:

# shared/side-repository.md
---
github-app:
  app-id: ${{ secrets.APP_ID }}
  private-key: ${{ secrets.APP_PRIVATE_KEY }}
---

Any workflow importing this file inherits the credentials without re-declaring them.

[Copilot]

Pull request overview

Updates the imports reference documentation to explicitly list github-app: as a supported top-level import field and to document its merge/precedence behavior when provided by shared imports.

Changes:

• Add github-app: to the “Allowed Import Fields” list for shared workflow imports.
• Document github-app: merge semantics in the “Field-Specific Merge Semantics” table (main workflow wins; otherwise first imported value is used).

Show a summary per file

File	Description
docs/src/content/docs/reference/imports.md	Documents github-app: as an allowed import field and clarifies its precedence/merge semantics.

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