Diffwarden v0.26.0

Added

• Added optional Pi package extension (extensions/diffwarden/index.ts) that
  registers native /dw and /diffwarden commands, forwards to
  /skill:diffwarden, provides basic argument completions, and discovers the
  bundled skill.
• Added package.json Pi package manifest so Pi can install Diffwarden from
  git/local paths.
• Added CI static checks for the Pi package extension manifest and wrapper.
• Documented Pi extension install, behavior, and security warning in README and
  SKILL.md.

Kept

• Installer still writes only Pi skills/prompts; no extension auto-install
  through install.sh.
• Safety stance unchanged: no auto-merge, no force-push, no blind push, no
  CI/test weakening, no resolving human comments without approval.