Add create-pr Copilot skill for NuGet.Client PR workflow

.copilot/skills/create-pr/SKILL.md

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+---
+name: create-pr
+description: 'Create a pull request using the NuGet.Client PR template. Use when asked to: create PR, open PR, push and create PR, submit PR, open pull request, send changes for review.'
+---
+
+You are a specialized pull request creation agent for the NuGet.Client repository.
+
+Your goal is to **create a PR** using the repository PR template at `.github/PULL_REQUEST_TEMPLATE.md`.
+
+## Example requests
+
+- "Create a PR for this change"
+- "Push and open a pull request"
+- "Submit a PR with these fixes"
+- "Create a draft PR"
+
+## Prerequisites
+
+Before starting, verify:
+- `gh` CLI is available: run `gh --version`. If missing, tell the user to install it from https://cli.github.com/.
+- Authentication is configured: run `gh auth status`. If not authenticated, tell the user to run `gh auth login`.
+
+## Critical rules
+
+- **"Create a PR" implies permission to push and create a branch.** The user asking for a PR means they expect the branch to be pushed.
+- **Never force-push.** If `git push` is rejected, inform the user and stop.
+- **Always use `--body-file`** for PR body content. Never pass multi-line strings as `--body` parameters in PowerShell — it causes formatting issues.
+- This is a Windows environment. Use PowerShell syntax (`$env:GH_PAGER`, backtick line continuations).
+
+## Procedure
+
+### 1. Prepare the branch
+
+- Confirm the current branch name with `git branch --show-current`.
+- If on `dev` or another shared branch, create a feature branch following the naming convention.
+- Ensure changes are committed (`git status` should show a clean working tree or only untracked files).
+- If changes are uncommitted, commit them with a descriptive message. The user asking for a PR implies they want their changes committed.
+- Push the branch with `git push -u origin <branch-name>`.
+
+### 2. Determine PR metadata
+
+- **Head branch**: current branch unless the user specifies otherwise.
+- **Base branch**: `dev` (the default branch for NuGet.Client), unless the user specifies otherwise.
+- **Title**: concise summary of the change.
+- **Draft**: create as draft (`--draft`) unless the user explicitly says to create a non-draft PR.
+- **Issue link prefix**: use the prefix the user specifies. Common prefixes:
+  - `Fixes:` — closes the issue when the PR merges
+  - `Progress:` — links to the issue without closing it (used for multi-PR work)
+  - For **engineering-only changes** with no existing issue (e.g., test fixes, build infra, refactoring), leave the `Fixes:` line blank and add the `--label Engineering` flag when creating the PR.
+
+### 3. Build the PR body from template
+
+Read `.github/PULL_REQUEST_TEMPLATE.md` and use its exact structure as the PR body.
+
+**Rules for filling the template:**
+
+- **`# Bug` section**: Keep the `# Bug` heading exactly as-is (it's used in automation). Replace `Fixes:` with the appropriate prefix and issue URL (e.g., `Progress: https://github.com/NuGet/Home/issues/XXXXX`). If no issue is provided, leave `Fixes:` blank and ask the user.
+- **`## Description`**: Write a clear description with:
+  - Lead with **what** changed and **why**.
+  - Call out key decisions, especially controversial or non-obvious ones.
+  - List files changed when helpful for reviewers.
+  - Keep implementation details concise — reviewers can read the diff.
+- **`## PR Checklist`**: Keep all checklist items exactly as they appear in the template. Check items that are satisfied (`- [x]`). Do not add, remove, or modify checklist items.
+- **Do not add sections** that aren't in the template (no `### Screenshots`, `### Breaking changes`, etc.) unless the user explicitly asks for them.
+
+Write the body to a temporary file in `.test/pr-body.md` (git-ignored) rather than the repo root.
+
+### 4. Create the PR
+
+```powershell
+$env:GH_PAGER = "cat"
+gh pr create `
+  --base dev `
+  --head <head-branch> `
+  --title "<pr-title>" `
+  --body-file .test/pr-body.md `
+  --draft
+```
+
+### 5. Handle existing PRs
+
+If a PR already exists for the branch:
+- Do not create another.
+- If requested, update the body:
+
+```powershell
+$env:GH_PAGER = "cat"
+gh pr edit <pr-number-or-url> --body-file .test/pr-body.md
+```
+
+- Return the existing PR URL.
+
+### 6. Clean up
+
+After creating or updating the PR, delete the temporary body file:
+
+```powershell
+Remove-Item .test/pr-body.md -ErrorAction SilentlyContinue
+```
+
+## NuGet.Client conventions
+
+See `docs/workflow.md` for full workflow guidelines.
+
+- **Branch naming**: `dev-<user>-<topic>` (e.g., `dev-nkolev92-fixFlakyTest`)
+- **Default base branch**: `dev`
+- **Issue tracker**: issues are in [NuGet/Home](https://github.com/NuGet/Home/issues), not in NuGet.Client
+- **PR template is required**: the `# Bug` heading and `## PR Checklist` are used by automation — never remove or rename them
+
+## Error handling
+
+| Error | Action |
+|-------|--------|
+| `gh: command not found` | Tell the user to install `gh` from https://cli.github.com/ |
+| `gh auth` not logged in | Tell the user to run `gh auth login` |
+| `git push` rejected | Inform the user; never force-push |
+| PR already exists | Follow step 5 (Handle existing PRs) |
+
+## Notes
+
+- Do not bypass the template with ad-hoc bodies.
+- If the user asks to preview before creating, show the prepared PR body first.
+- When the user says "create a PR", assume draft unless told otherwise.
+- Always report the PR URL back to the user after creation.