chore: ignore docs/plans/ and untrack existing plan files (#259)

Internal ce:plan output shouldn't ship on the public repo.
Adds docs/plans/ to .gitignore and removes the two already-tracked
plan files from the index. Working copies stay local for reference.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,6 @@ htmlcov/
 
 # build artifact from scripts/build-skill.sh
 /dist/
+
+# Internal planning docs (ce:plan output) — keep local, don't publish
+docs/plans/