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/save_antigravity_session.fish

@@ -0,0 +1,32 @@
+#!/usr/bin/env fish
+
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# 1. Read the JSON from stdin
+set -l input (cat)
+
+# 2. Extract session_id using Python
+set -l sid (echo $input | python3 -c "import sys,json; print(json.load(sys.stdin).get('session_id', ''))")
+set -l session_file ".antigravity_session"
+
+if test -n "$sid"
+    # 3. Save the session ID locally
+    echo "$sid" >$session_file
+
+    # 4. Smart .gitignore check
+    # We only attempt this if we are inside a Git repository
+    if git rev-parse --is-inside-work-tree >/dev/null 2>&1
+        # If 'git check-ignore' fails, it means the file is NOT currently ignored
+        if not git check-ignore -q $session_file
+            # Append a labeled entry to .gitignore
+            echo -e "\n# AI Session IDs\n$session_file" >>.gitignore
+        end
+    end
+
+    # 5. Update universal variable for cross-terminal access
+    set -U LAST_ANTIGRAVITY_SESSION "$sid"
+end
+
+# MANDATORY: Every hook must output valid JSON or an empty object
+echo '{}'