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/antigravity-ide.fish

@@ -0,0 +1,8 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# alias antigravity-ide=antigravity
+function antigravity-ide --wraps='antigravity-ide' --description 'alias antigravity-ide=antigravity-ide'
+    # In fish, we pipe stderr using '2>|' to another command
+    command antigravity-ide $argv 2>| grep -v "'app' is not in the list of known options" >&2
+end