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)

docs/mitm.md

@@ -132,11 +132,11 @@ Events dispatched through `tokio::sync::mpsc` channels from MITM → API handler
 ### UID-Scoped iptables (Classic Mode)
 
 ```bash
-# One-time setup — creates antigravity-ls user + iptables rule
+# One-time setup — creates zerogravity-ls user + iptables rule
 sudo ./scripts/mitm-redirect.sh install
 
 # Run proxy (standalone LS + MITM both enabled by default)
-RUST_LOG=info ./target/release/antigravity-proxy
+RUST_LOG=info ./target/release/zerogravity
 
 # Check intercepted usage
 curl -s http://localhost:8741/v1/usage | jq .
@@ -150,7 +150,7 @@ sudo ./scripts/mitm-redirect.sh uninstall
 No iptables or sudo needed. The LS connects through `HTTPS_PROXY` instead:
 
 ```bash
-RUST_LOG=info ./target/release/antigravity-proxy --headless
+RUST_LOG=info ./target/release/zerogravity --headless
 ```
 
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