fix(agents-init): migrate plugin dirs, smarter CLAUDE.md handling, wrappers run full init

- Move plugin dir creation into --plugins mode (not 'Always') so --agents
  alone no longer creates empty AGENTS/plugins/ dirs
- Wrappers (claude, agy) now call plain agents-init (both modes) so plugin
  dirs in docs/ are migrated on the first AI launch, not just --agents
- Rewrite --agents CLAUDE.md logic: detect which root files are real vs
  symlinks and handle all four cases (both/agents-only/claude-only/neither)
  without ever losing content
- AGENTS/CLAUDE.md is now its own file in AGENTS/ (real file when both
  existed, symlink → AGENTS.md for single-source cases)
- Root CLAUDE.md now points to AGENTS/CLAUDE.md instead of AGENTS/AGENTS.md;
  readlink checks auto-fix existing symlinks with the wrong target
- New empty setups get AGENTS/AGENTS.md pre-populated with the agent
  directive warning editors not to write through the root symlinks

functions/agy.fish

@@ -8,7 +8,7 @@
 #   Wrapper for the agy Antigravity AI CLI that ensures the AGENTS/
 #   sub-repository is initialized and any agent-made changes are committed
 #   before launch. Delegates all scaffold and commit logic to agents-init
-#   --agents. All arguments are forwarded verbatim to the real agy binary.
+#   (full setup). All arguments are forwarded verbatim to the real agy binary.
 #
 #   Opinionated component (C1): when disabled via __fish_config_op_aliases
 #   (or the __fish_config_opinionated master), the command is passed through
@@ -30,7 +30,7 @@ function agy --wraps=agy --description 'agy wrapper: auto-initializes AGENTS/ su
         return $status
     end
 
-    agents-init --agents
+    agents-init
 
     command agy $argv
 end