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)

scripts/mitm-redirect.sh

@@ -6,7 +6,7 @@
 # modification, no system-wide changes.
 #
 # Flow:
-#   1. Standalone LS runs as 'antigravity-ls' user (via sudo -u)
+#   1. Standalone LS runs as 'zerogravity-ls' user (via sudo -u)
 #   2. iptables catches :443 traffic from that UID only → REDIRECT to MITM port
 #   3. MITM terminates TLS (Go client trusts our CA via SSL_CERT_FILE)
 #   4. MITM forwards upstream, captures usage
@@ -24,10 +24,10 @@
 set -euo pipefail
 
 MITM_PORT="${2:-8742}"
-LS_USER="antigravity-ls"
+LS_USER="zerogravity-ls"
 DATA_DIR="/tmp/antigravity-standalone"
 LS_BINARY="/usr/share/antigravity/resources/app/extensions/antigravity/bin/language_server_linux_x64"
-SUDOERS_FILE="/etc/sudoers.d/antigravity-ls"
+SUDOERS_FILE="/etc/sudoers.d/zerogravity-ls"
 
 install() {
     if [[ $EUID -ne 0 ]]; then
@@ -54,7 +54,7 @@ install() {
     echo "  + data dir: $DATA_DIR (mode 1777, writable by all)"
 
     # ── 3. Sudoers entry ────────────────────────────────────────────────
-    # Allow the invoking user (SUDO_USER) to run ANY command as antigravity-ls.
+    # Allow the invoking user (SUDO_USER) to run ANY command as zerogravity-ls.
     # This is needed for the proxy to spawn the LS binary.
     local REAL_USER="${SUDO_USER:-$(logname 2>/dev/null || whoami)}"
     cat > "$SUDOERS_FILE" <<EOF
@@ -78,7 +78,7 @@ EOF
     echo
     echo "[mitm-redirect] ✓ Installed (only affects uid=$LS_UID)"
     echo "  Restart the proxy to take effect:"
-    echo "    RUST_LOG=info ./target/release/antigravity-proxy --standalone"
+    echo "    RUST_LOG=info ./target/release/zerogravity --standalone"
 }
 
 uninstall() {