Removes development-workflow artifacts that aren't authoritative
docs and go stale as the code evolves:

- docs/superpowers/ (~730 KB) — brainstorming specs + implementation
  plans written by the superpowers skills during Block 1-7 work. The
  durable record lives in commit messages and PR descriptions.
- docs/history/ (~60 KB) — older retrospective reviews (REVIEW,
  POST-SWEEP-REVIEW, UNIFICATION-PLAN) from pre-roadmap sweeps.

Adds docs/superpowers/ to .gitignore so future plan output stays
local without having to manually clean before each commit.

No links in the tree point at either directory, so nothing else
needs updating. User-facing docs (architecture, config, rest-api,
mcp-tools, hooks, cli-reference, quickstart, getting-started,
ACCESSIBILITY, samples/, screenshots/) are untouched.

Net: -22000 LOC across 16 files; no runtime impact.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>