task md:lint corrupts the .github/copilot-instructions.md symlink on Windows

### What happened

Running the documented `task md:lint` on Windows modifies `.github/copilot-instructions.md`, which is a symlink to `../AGENTS.md`. Committing that change would replace the symlink with a broken target path and silently detach the repository's Copilot instructions from `AGENTS.md`.

### Why it happens

`.github/copilot-instructions.md` is stored as a symlink (mode `120000`) whose blob content is the literal string `../AGENTS.md` with no trailing newline.

Git for Windows defaults to `core.symlinks=false`. Under that setting the entry is checked out as a plain text file containing the target path rather than a real symlink. `markdownlint-cli2` then sees an ordinary `.md` file that does not end in a newline and "fixes" it:

```text
.github/copilot-instructions.md:1:12 error MD047/single-trailing-newline Files should end with a single newline character
```

The resulting diff against a clean tree:

```diff
diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index be77ac8..6afa560 120000
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -1 +1 @@
-../AGENTS.md
\ No newline at end of file
+../AGENTS.md
```

The mode stays `120000`, so the entry is still a symlink as far as Git is concerned, but its target becomes `"../AGENTS.md\n"` — a path that does not exist.

### Reproduction

On Windows with `core.symlinks=false` (the default):

```powershell
git clone https://github.com/Azure/mpf.git
cd mpf
task md:tools
task md:lint
git status --short # M .github/copilot-instructions.md
```

Isolated to confirm which tool is responsible:

```powershell
task md:run:markdownlint-cli2:fix --force
git status --short # M .github/copilot-instructions.md
```

`markdown-table-formatter` does not touch the file. `markdownlint-cli2` is the one that rewrites it.

### Why it is easy to miss

The change is a single invisible trailing newline in a one line file. Anyone who runs the documented lint task and then stages with `git add -A` or `git commit -a` will pick it up without noticing, and the diff looks harmless in review. I hit this while fixing an unrelated markdown table in #314 and only caught it because the file was outside the scope of my change.

### Suggested fix

Exclude the symlink from linting in `.github/linters/.markdownlint-cli2.yaml`:

```yaml
ignores:
- .git
- "**/node_modules/**"
- .copilot-tracking/**
- venv/**
- .venv/**
- .github/copilot-instructions.md
```

That keeps the current symlink layout, which is nice because `AGENTS.md` stays the single source of truth.

Alternatives, if you would prefer not to special case a path:

- Disable `MD047` for that file via an inline configuration comment, though that is awkward since any content added to a symlink defeats its purpose.
- Replace the symlink with a small stub file that points readers at `AGENTS.md`, avoiding symlink portability issues altogether at the cost of duplication.

The ignore entry seems like the smallest change.

### Environment

- Windows, Git for Windows with `core.symlinks=false`
- `markdownlint-cli2` v0.23.1 (markdownlint v0.41.1), as pinned by `task md:install:markdownlint-cli2`
- `main` as of 2026-07-27

This is not specific to my checkout. Any Windows contributor with default Git settings who runs the documented lint task will reproduce it.