Update Copilot guidance and PR prompt workflow (This PR summary and commit are generated accordingly) (#42634)

## Summary of the Pull Request
- Added new Copilot agent prompts in
`.github/prompts/create-commit-title.prompt.md`,
`.github/prompts/create-pr.prompt.md`, and
`.github/prompts/fix-spelling.prompt.md` to streamline contributor
workflows.
- Refreshed `.github/copilot-instructions.md` with front matter and
clarified build/test guidance.
And added a new section titled `Language Style Rules` to help reduce
style issues in auto-generated code.

## Screenshot
a. For the new prompts, in VSCode, just type "/" and its name
<img width="477" height="124" alt="image"
src="https://github.com/user-attachments/assets/37c7330d-6fdc-45b4-9030-95920920c964"
/>
<img width="446" height="105" alt="image"
src="https://github.com/user-attachments/assets/55d3bb49-46cc-4441-9fb5-fed3a22983d9"
/>
b. All the spelling check problem, PR description, Commit title, are
generated and fixed by above prompts as demonstration (Except this
section)

## PR Checklist
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** N/A on doc updates.
- [x] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx (N/A)

## Detailed Description of the Pull Request / Additional comments
- Introduced prompt definitions so local tooling can generate commit
titles, PR descriptions, and resolve spell-check alerts using consistent
instructions.
- Added GitHub CLI prerequisites inside
`.github/prompts/fix-spelling.prompt.md` to ensure `gh` commands succeed
for new contributors.
- Converted `.github/copilot-instructions.md` to YAML-front-matter
format while aligning headings and terminology with current contributor
guidance.

## Validation Steps Performed
- No automated tests were run; changes are documentation and
configuration only.

.github/prompts/create-pr-summary.prompt.md

@@ -0,0 +1,22 @@
+---
+mode: 'agent'
+model: GPT-5-Codex (Preview)
+description: 'Generate a PowerToys-ready pull request description from the local diff.'
+---
+
+**Goal:** Produce a ready-to-paste PR title and description that follows PowerToys conventions by comparing the current branch against a user-selected target branch.
+
+**Repo guardrails:**
+- Treat `.github/pull_request_template.md` as the single source of truth; load it at runtime instead of embedding hardcoded content in this prompt.
+- Preserve section order from the template but only surface checklist lines that are relevant for the detected changes, filling them with `[x]`/`[ ]` as appropriate.
+- Cite touched paths with inline backticks, matching the guidance in `.github/copilot-instructions.md`.
+- Call out test coverage explicitly: list automated tests run (unit/UI) or state why they are not applicable.
+
+**Workflow:**
+1. Determine the target branch from user context; default to `main` when no branch is supplied.
+2. Run `git status --short` once to surface uncommitted files that may influence the summary.
+3. Run `git diff <target-branch>...HEAD` a single time to review the detailed changes. Only when confidence stays low dig deeper with focused calls such as `git diff <target-branch>...HEAD -- <path>`.
+4. From the diff, capture impacted areas, key file changes, behavioral risks, migrations, and noteworthy edge cases.
+5. Confirm validation: list tests executed with results or state why tests were skipped in line with repo guidance.
+6. Load `.github/pull_request_template.md`, mirror its section order, and populate it with the gathered facts. Include only relevant checklist entries, marking them `[x]/[ ]` and noting any intentional omissions as "N/A".
+7. Present the filled template inside a fenced ```markdown code block with no extra commentary so it is ready to paste into a PR, clearly flagging any placeholders that still need user input.