docs: retire superpowers/ layout — keep design spec under docs/specs/

The superpowers workflow methodology is no longer in use. Move the
methodology-neutral design spec to docs/specs/ as the project's design
ledger, drop the superpowers-bound implementation plan (it referenced
superpowers sub-skills), and remove the now-dead /docs/superpowers/
.gitignore entry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -11,5 +11,3 @@
 # Lokale, persönliche Claude-Code-Overrides (nicht teamweit)
 /.claude/settings.local.json
 /.claude/worktrees/
-
-/docs/superpowers/