docs: clean up AI assistant alias config

Unignore CLAUDE.md and .agent, add CLAUDE.md alias file,
and simplify the developer guide alias section.

.gitignore

@@ -37,8 +37,6 @@ package.json.bak
 .wxt
 
 # AI
-.agent
 .claude
 .cursor
-.gemini
+.gemini
-CLAUDE.md

CLAUDE.md

@@ -0,0 +1 @@
+@AGENTS.md

docs/developer-guide.md

@@ -44,23 +44,7 @@ Applications:
 
 ## 🤖 AGENTS.md Alias
 
-If your AI assistant does not support [AGENTS.md](https://agents.md/). Add an alias for it:
+If your AI assistant does not support [AGENTS.md](https://agents.md/). Add an alias for it.
-
-- claude-code (`CLAUDE.md`)
-
-    ```markdown
-    @AGENTS.md
-    ```
-
-- antigravity (`.agent/rules/alias.md`)
-
-    ```markdown
-    ---
-    trigger: always_on
-    ---
-
-    @../../AGENTS.md
-    ```
 
 ## 🔧 Development Workflows