feat(gemini): add Gemini CLI extension support

Add gemini-extension.json manifest with correct array-format settings,
symlink skills/last30days/SKILL.md to root SKILL.md for Gemini skill
discovery, add Gemini install paths to bash for-loop in both main and
open variant, and add Gemini CLI install instructions to README.

Incorporates the good parts of PR #53 (manifest, paths, README) while
avoiding duplicate SKILL.md, tool name scattering, and allowed-tools
pollution that would have created maintenance issues.

Closes #45

Co-Authored-By: Alex Ferrari <alex@thealexferrari.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

variants/open/SKILL.md

@@ -27,6 +27,9 @@ Parse the user's first argument to determine the mode:
 for dir in \
   "." \
   "${CLAUDE_PLUGIN_ROOT:-}" \
+  "${GEMINI_EXTENSION_DIR:-}" \
+  "$HOME/.gemini/extensions/last30days-skill" \
+  "$HOME/.gemini/extensions/last30days" \
   "$HOME/.claude/skills/last30days" \
   "$HOME/.agents/skills/last30days" \
   "$HOME/.codex/skills/last30days"; do