feat: rebrand to ZeroGravity, replace proxyctl with zg Rust binary

Phase 1 - Rename:
- Crate: antigravity-proxy -> zerogravity
- Env: ANTIGRAVITY_OAUTH_TOKEN -> ZEROGRAVITY_TOKEN
- Paths: ~/.config/antigravity-proxy -> ~/.config/zerogravity
- Paths: /tmp/antigravity-* -> /tmp/zerogravity-*
- User: antigravity-ls -> zerogravity-ls
- Service: antigravity-proxy -> zerogravity

Phase 2 - zg daemon manager:
- New Rust binary src/bin/zg.rs replaces scripts/proxyctl bash
- Commands: start, stop, restart, rebuild, status, logs, test, health
- Auto-resolves project dir from binary location
- All commands exit immediately (safe for agent fast-bash)

src/constants.rs

@@ -185,7 +185,7 @@ pub fn log_base() -> String {
 /// Token file path.
 pub fn token_file_path() -> String {
     let home = std::env::var("HOME").unwrap_or_else(|_| "/root".to_string());
-    format!("{home}/.config/antigravity-proxy-token")
+    format!("{home}/.config/zerogravity/token")
 }
 
 /// User-Agent string matching the Electron webview — computed once.