chore: gitignore docs_internal and tai-kernel (internal harness/notes/kernels)

These hold the internal benchmarking harness, investigation notes, and local
kernel sources used during development. They were inadvertently committed in
earlier lever-A work and have been stripped from history; ignore them so they
stay on disk for local use but never get tracked again.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -275,3 +275,8 @@ sgl-kernel/csrc/**/*_musa/
 
 # Git worktrees
 .worktrees/
+
+# Internal benchmarking harness, investigation notes, and local kernels
+# (kept on disk for local work, never committed)
+docs_internal/
+tai-kernel/