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)

Cargo.toml

@@ -1,8 +1,16 @@
 [package]
-name = "antigravity-proxy"
+name = "zerogravity"
 version = "3.0.0"
 edition = "2021"
 
+[[bin]]
+name = "zerogravity"
+path = "src/main.rs"
+
+[[bin]]
+name = "zg"
+path = "src/bin/zg.rs"
+
 [dependencies]
 axum = { version = "0.8", features = ["json"] }
 tokio = { version = "1", features = ["full"] }