chore: gitignore docs/superpowers/ — internal scratch should not ship (#128)

The folder holds workspace research, plans, and specs that contain
unredacted incident detail, customer references, and intermediate
reasoning. None of it is part of the published documentation; it lives
in `docs/` only because the maintainer's tooling co-locates working
notes with the project. Move it out of git's view:

- Add `docs/superpowers/` to `.gitignore`.
- `git rm -r --cached docs/superpowers/` to untrack the 18 files
  (kept on disk for the maintainer; they just stop being tracked).
- Replace the dangling docstring reference in `lance_maintenance.py`
  that pointed at one of the removed files with a self-contained
  description plus a pointer to the public RFC-009 §Phase 1.5 section.

CHANGELOG.md and the public RFC retain a few historical breadcrumbs to
file paths under `docs/superpowers/`. Those references read as
"see internal research" and don't leak content; left in place so the
git history of the docs we *do* publish is faithful. Future doc
authors should prefer linking to RFC sections or the public CHANGELOG
entry rather than to research files.

Author: rolandpg

.gitignore

@@ -12,6 +12,17 @@ tasks/
 docs/.ralph/
 agentsync.md
 
+# Internal research/specs/plans — workspace scratch that contains
+# unredacted incident detail, customer references, and intermediate
+# reasoning. Kept on disk for the maintainer; not published.
+docs/superpowers/
+
+# Claude Code session artifacts — agent worktrees, transcripts, and
+# scratch state created by the maintainer's local tooling. Belt-and-
+# suspenders: these were never tracked, but listing them prevents an
+# accidental `git add -A` from sweeping them in.
+.claude/
+
 # MkDocs build output (regenerated from docs/)
 site/