chore: remove plan docs and gitignore docs/plans/ (heygen-com#1265)

Plan documents are working artifacts that shouldn't be committed.
Removes three plans that were accidentally tracked and adds
docs/plans/ to .gitignore to prevent future occurrences.

Author: miguel-heygen

.gitignore

@@ -66,6 +66,9 @@ tmp/
 packages/core/src/generated/
 packages/producer/src/services/fontData.generated.ts
 
+# Plan documents (working artifacts, not committed)
+docs/plans/
+
 # Local proof / test artifacts
 qa-artifacts/
 my-video/