Mark test snapshot files as linguist-generated

- Add comprehensive patterns to .gitattributes to exclude test snapshot files from GitHub language statistics
- Covers __snapshots__, snapshots, .snapshots, _snapshots directories
- Includes .snap and .snapshot file extensions
- Affects 27 existing snapshot files in src/core/prompts/__tests__/__snapshots__/
- Future-proofs against new snapshot files from testing frameworks like Vitest and Jest

Resolves Slack request to mark snapshot files as linguist-generated=true

Author: roomote

.gitattributes

@@ -1,3 +1,11 @@
 demo.gif filter=lfs diff=lfs merge=lfs -text
 assets/docs/demo.gif filter=lfs diff=lfs merge=lfs -text
 src/assets/docs/demo.gif filter=lfs diff=lfs merge=lfs -text
+
+# Test snapshot files - mark as linguist-generated to exclude from GitHub language statistics
+**/__snapshots__/** linguist-generated=true
+**/snapshots/** linguist-generated=true
+**/.snapshots/** linguist-generated=true
+**/_snapshots/** linguist-generated=true
+*.snap linguist-generated=true
+*.snapshot linguist-generated=true