fix: fetch PR head history for Lunaria builds

[IEvangelist]

Summary

Follow up to #707 by fetching the PR head ref in the reusable frontend workflow before the Lunaria build.

Why

The earlier git fetch --unshallow change was necessary, but not sufficient for all pull request checkouts. We still have failing frontend-build runs where Lunaria logs:

[info] Shallow repository detected. A clone of your repository's history will be downloaded and used.

and then fails on a newly added docs page such as:

• /src/frontend/src/content/docs/get-started/faq.mdx in Add FAQ page and homepage FAQ link #701
• /src/frontend/src/content/docs/deployment/environments.mdx in Add environments documentation guide #705

Root cause

On some PR merge checkouts (refs/pull/<n>/merge), Lunaria still sees the repository as shallow and switches into its fallback history path. That path cannot resolve git history for docs pages that only exist on the PR branch.

Fix

When this reusable workflow is running for a pull request, fetch:

refs/pull/<n>/head

explicitly, with --unshallow, before the frontend build. This makes the PR head history available even when the checkout was created from the merge ref.

For non-PR builds, keep the existing git fetch --unshallow fallback.

Validation

Simulated PR merge checkouts for the known failing cases:

• PR Add FAQ page and homepage FAQ link #701: shallow true -> false, faq.mdx history restored
• PR Add environments documentation guide #705: shallow true -> false, environments.mdx history restored

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the reusable frontend-build workflow to ensure Lunaria can access PR head history during PR merge checkouts by explicitly fetching the PR head ref before building.

Changes:

• Add an optional pull_number input to the reusable frontend-build workflow.
• For PR runs, fetch refs/pull/<n>/head (attempting --unshallow first) to make PR head history available to Lunaria.
• Pass the PR number from ci.yml into the reusable workflow when the event is pull_request.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/frontend-build.yml	Adds pull_number input and PR-aware git fetch logic to ensure Lunaria sees PR head history.
.github/workflows/ci.yml	Wires github.event.pull_request.number into the reusable frontend workflow for PR runs.

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[IEvangelist]

Addressed the review feedback in the latest push:

• validate pull_number as numeric before using it in the refspec
• stop swallowing real git fetch failures
• use a clearer refs/remotes/origin/pull/<n>/head destination ref and store the refspec in a variable

The guarded fetch path still reproduces correctly against the known failing PR shapes (#701 and #705): shallow true -> false, and the new docs page history is available after the fetch.