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)

README.md

@@ -23,10 +23,10 @@ graph LR
 
 ```bash
 # Headless mode (no running Antigravity app needed)
-RUST_LOG=info ./target/release/antigravity-proxy --headless
+RUST_LOG=info ./target/release/zerogravity --headless
 
 # Or use the daemon manager
-proxyctl start
+zg start
 ```
 
 ## Endpoints
@@ -50,22 +50,22 @@ proxyctl start
 
 The proxy needs an OAuth token:
 
-1. **Env var**: `ANTIGRAVITY_OAUTH_TOKEN=ya29.xxx`
-2. **Token file**: `~/.config/antigravity-proxy-token`
+1. **Env var**: `ZEROGRAVITY_TOKEN=ya29.xxx`
+2. **Token file**: `~/.config/zerogravity-token`
 3. **Runtime**: `curl -X POST http://localhost:8741/v1/token -d '{"token":"ya29.xxx"}'`
 
-## `proxyctl` Commands
+## `zg` Commands
 
 | Command               | Description                    |
 | --------------------- | ------------------------------ |
-| `proxyctl start`      | Start the proxy daemon         |
-| `proxyctl stop`       | Stop the proxy daemon          |
-| `proxyctl restart`    | Rebuild + restart              |
-| `proxyctl rebuild`    | Build release binary only      |
-| `proxyctl status`     | Service status + quota + usage |
-| `proxyctl logs [N]`   | Tail last N lines + follow     |
-| `proxyctl test [msg]` | Quick test request             |
-| `proxyctl health`     | Health check                   |
+| `zg start`      | Start the proxy daemon         |
+| `zg stop`       | Stop the proxy daemon          |
+| `zg restart`    | Rebuild + restart              |
+| `zg rebuild`    | Build release binary only      |
+| `zg status`     | Service status + quota + usage |
+| `zg logs [N]`   | Tail last N lines + follow     |
+| `zg test [msg]` | Quick test request             |
+| `zg health`     | Health check                   |
 
 ## Documentation