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/fix-spelling.prompt.md

@@ -0,0 +1,22 @@
+---
+mode: 'agent'
+model: GPT-5-Codex (Preview)
+description: 'Resolve Code scanning / check-spelling comments on the active PR.'
+---
+
+**Goal:** Clear every outstanding GitHub pull request comment created by the `Code scanning / check-spelling` workflow by explicitly allowing intentional terms.
+
+**Guardrails:**
+- Update only discussion threads authored by `github-actions` or `github-actions[bot]` that mention `Code scanning results / check-spelling`.
+- Resolve findings solely by editing `.github/actions/spell-check/expect.txt`; reuse existing entries.
+- Leave all other files and topics untouched.
+
+**Prerequisites:**
+- Install GitHub CLI if it is not present: `winget install GitHub.cli`.
+- Run `gh auth login` once before the first CLI use.
+
+**Workflow:**
+1. Determine the active pull request with a single `gh pr view --json number` call (default to the current branch).
+2. Fetch all PR discussion data once via `gh pr view --json comments,reviews` and filter to check-spelling comments authored by `github-actions` or `github-actions[bot]` that are not minimized; when several remain, process only the most recent comment body.
+3. For each flagged token, review `.github/actions/spell-check/expect.txt` for an equivalent term (for example an existing lowercase variant); when found, reuse that normalized term rather than adding a new entry, even if the flagged token differs only by casing. Only add a new entry after confirming no equivalent already exists.
+4. Add any remaining missing token to `.github/actions/spell-check/expect.txt`, keeping surrounding formatting intact.