feat(ai): migrate gemini-cli references to antigravity-cli (agy)

Rename gemini-resume → antigravity-resume and save_gemini_session →
save_antigravity_session; update all session files from .gemini_session
to .antigravity_session and LAST_GEMINI_SESSION → LAST_ANTIGRAVITY_SESSION.
Replace gemini/gemini-cli binary calls with agy throughout superpowers,
code-resume, and the resume function. Add antigravity-ide wrapper for the
renamed IDE binary; update abbr v → antigravity-ide (VSCode-equivalent)
and update README/requirements to reflect the new tool names.

functions/code-resume.fish

@@ -7,10 +7,10 @@ function code-resume --description 'Execute code-resume'
         set -l sid (cat .claude_session)
         echo "Resuming Claude session: $sid"
         claude --resume $sid
-    else if test -f .gemini_session
-        set -l sid (cat .gemini_session)
-        echo "Resuming Gemini session: $sid"
-        gemini --resume $sid
+    else if test -f .antigravity_session
+        set -l sid (cat .antigravity_session)
+        echo "Resuming antigravity-cli session: $sid"
+        agy --resume $sid
     else
         echo "No local AI session found. Opening picker..."
         claude --resume # Default to Claude picker