docs: add IDEAS.md for informal backlog, link from CLAUDE.md

A lightweight, low-friction place to jot future development ideas before
they're formalized into a spec/plan -- distinct from
docs/superpowers/specs/ and docs/superpowers/plans/, which are for once an
idea is ready to be built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/IDEAS.md

@@ -0,0 +1,15 @@
+# Ideas for future development
+
+Informal running list of things worth doing eventually. Not specs, not plans —
+just enough to jog memory at the start of a future session. When one of these
+is actually ready to be built, run it through the normal brainstorming ->
+writing-plans flow (see `docs/superpowers/specs/` and `docs/superpowers/plans/`
+for that history) and remove it from here once a spec exists.
+
+## Backlog
+
+- (add ideas here)
+
+## Someday / maybe
+
+- (lower-priority or vaguer ideas here)