docfx: sync from repo-template (gh-pages bootstrap fix)

[Chris-Wolfgang]

Summary

Sync .github/workflows/docfx.yaml from repo-template to pick up the gh-pages bootstrap fix from Chris-Wolfgang/repo-template#337.

What changed

The "Deploy docs to GitHub Pages" step had a latent bug: when the gh-pages branch did not yet exist on the remote, $WORK_DIR was created as a plain directory and the subsequent git add / diff --cached / commit / push failed with fatal: not a git repository. The fix initializes the directory as a git repo on gh-pages and adds the authenticated origin remote, so the existing add/commit/push works on a first-ever deploy. Cleanup also branches between git worktree remove (worktree path) and Remove-Item (fresh-repo path).

If gh-pages already exists in this repo (the common case), this change is a no-op at runtime — the bootstrap branch is only taken on a fresh repo.

Test plan

• Workflow YAML lints / next docs deploy succeeds
• (Optional) verify gh-pages is unaffected

[Copilot]

Pull request overview

Note

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

Syncs the DocFX GitHub Pages deployment workflow with the template to fix first-time deployments when gh-pages doesn’t exist yet, by bootstrapping a valid git repo in $WORK_DIR and adjusting cleanup behavior.

Changes:

• Add a “fresh repo” bootstrap path that runs git init on gh-pages and adds origin when the remote branch doesn’t exist.
• Track branch existence via $useWorktree and branch cleanup between git worktree remove vs Remove-Item.
• Explicitly reset $LASTEXITCODE after cleanup to avoid step failures due to cleanup commands.

[Chris-Wolfgang]

Superseded by the canonical maintenance baseline #64. The gh-pages bootstrap fix from repo-template#337 (init the work dir as a git repo on gh-pages when the branch doesn't exist, branch cleanup between git worktree remove and Remove-Item) is already present in vNext's docfx.yaml (git init --initial-branch=gh-pages + branchExists logic). Nothing left to apply — closing.