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/api/completions.rs

@@ -390,7 +390,7 @@ pub(crate) async fn handle_completions(
     if token.is_empty() {
         return err_response(
             StatusCode::UNAUTHORIZED,
-            "No OAuth token. POST to /v1/token or set ANTIGRAVITY_OAUTH_TOKEN env var.".into(),
+            "No OAuth token. POST to /v1/token or set ZEROGRAVITY_TOKEN env var.".into(),
             "authentication_error",
         );
     }