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.lock

@@ -103,43 +103,6 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
-[[package]]
-name = "antigravity-proxy"
-version = "3.0.0"
-dependencies = [
- "async-stream",
- "axum",
- "base64",
- "brotli 7.0.0",
- "bytes",
- "chrono",
- "clap",
- "flate2",
- "http",
- "http-body-util",
- "httparse",
- "hyper",
- "hyper-util",
- "rand",
- "rcgen",
- "regex",
- "rustls",
- "rustls-native-certs",
- "rustls-pemfile",
- "serde",
- "serde_json",
- "time",
- "tokio",
- "tokio-rustls",
- "tokio-stream",
- "tower-http",
- "tracing",
- "tracing-subscriber",
- "uuid",
- "wreq",
- "wreq-util",
-]
-
 [[package]]
 name = "async-stream"
 version = "0.3.6"
@@ -2396,6 +2359,43 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "zerogravity"
+version = "3.0.0"
+dependencies = [
+ "async-stream",
+ "axum",
+ "base64",
+ "brotli 7.0.0",
+ "bytes",
+ "chrono",
+ "clap",
+ "flate2",
+ "http",
+ "http-body-util",
+ "httparse",
+ "hyper",
+ "hyper-util",
+ "rand",
+ "rcgen",
+ "regex",
+ "rustls",
+ "rustls-native-certs",
+ "rustls-pemfile",
+ "serde",
+ "serde_json",
+ "time",
+ "tokio",
+ "tokio-rustls",
+ "tokio-stream",
+ "tower-http",
+ "tracing",
+ "tracing-subscriber",
+ "uuid",
+ "wreq",
+ "wreq-util",
+]
+
 [[package]]
 name = "zeroize"
 version = "1.8.2"