Add PublicAPI.Unshipped.txt BOM sort warning to Copilot instructions (dotnet#34327)

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## Description

PR dotnet#34320 fixed RS0017 analyzer errors caused by `#nullable enable`
being sorted to the bottom of 14 Maps `PublicAPI.Unshipped.txt` files.
The root cause was a prior Copilot agent session that used `LC_ALL=C
sort -u` to resolve merge conflicts — the BOM bytes (`0xEF 0xBB 0xBF`)
sort after all ASCII characters, pushing the directive below the API
entries.

This updates the Copilot instructions to prevent this from recurring:

- Explains that `#nullable enable` must remain on line 1
- Warns against using plain `sort` on these files (BOM sort ordering)
- Provides a safe conflict resolution script that preserves the header
before sorting API entries

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jfversluis

.github/copilot-instructions.md

@@ -138,6 +138,26 @@ When working with public API changes:
 - **Use `dotnet format analyzers`** if having trouble
 - **If files are incorrect**: Revert all changes, then add only the necessary new API entries
 
+**🚨 CRITICAL: `#nullable enable` must be line 1**
+
+Every `PublicAPI.Unshipped.txt` file starts with `#nullable enable` (often BOM-prefixed: `#nullable enable`) on the **first line**. If this line is moved or removed, the analyzer treats it as a declared API symbol and emits **RS0017** errors.
+
+**Never sort these files with plain `sort`** — the BOM bytes (`0xEF 0xBB 0xBF`) sort after ASCII characters under `LC_ALL=C`, pushing `#nullable enable` to the bottom of the file.
+
+When resolving merge conflicts or adding entries, use this safe pattern that preserves line 1:
+```bash
+for f in $(git diff --name-only --diff-filter=U | grep "PublicAPI.Unshipped.txt"); do
+  # Extract and preserve the #nullable enable line (with or without BOM)
+  HEADER=$(head -1 "$f" | grep -o '.*#nullable enable' || echo '#nullable enable')
+  # Strip conflict markers, remove all #nullable lines, sort+dedup the API entries
+  grep -v '^<<<<<<\|^======\|^>>>>>>\|#nullable enable' "$f" | LC_ALL=C sort -u | sed '/^$/d' > /tmp/api_fix.txt
+  # Reassemble: header first, then sorted entries
+  printf '%s\n' "$HEADER" > "$f"
+  cat /tmp/api_fix.txt >> "$f"
+  git add "$f"
+done
+```
+
 ### Branching
 - `main` - For bug fixes without API changes
 - `net10.0` - For new features and API changes