Add Copilot review instructions for OSS license files (#300954)

.github/instructions/oss.instructions.md

@@ -0,0 +1,34 @@
+---
+applyTo: '{ThirdPartyNotices.txt,cli/ThirdPartyNotices.txt,cglicenses.json,cgmanifest.json}'
+---
+
+# OSS License Review
+
+When reviewing changes to these files, verify:
+
+## ThirdPartyNotices.txt
+
+- Every new entry has a license type header (e.g., "MIT License", "Apache License 2.0")
+- License text is present and non-empty for every entry
+- License text matches the declared license type (e.g., MIT-declared entry actually contains MIT license text, not Apache)
+- Removed entries are cleanly removed (no leftover fragments)
+- Entries are sorted alphabetically by package name
+
+## cglicenses.json
+
+- New overrides have a justification comment
+- No obviously stale entries for packages no longer in the dependency tree
+
+## cgmanifest.json
+
+- Package versions match what's actually installed
+- Repository URLs are valid and point to real source repositories
+- Newly added license identifiers should use SPDX format where possible
+- License identifiers match the corresponding ThirdPartyNotices.txt entries
+
+## Red Flags
+
+- Any **newly added** copyleft license (GPL, LGPL, AGPL) — flag immediately (existing copyleft entries like ffmpeg are pre-approved)
+- Any "UNKNOWN" or placeholder license text
+- License text that appears truncated or corrupted
+- A package declared as MIT but with Apache/BSD/other license text (or vice versa)