refactor: Remove castle sync command

Sync was a bootstrap command that ran `git submodule update` and `uv tool install` for each program. Neither machine uses git submodules anymore, and the install functionality is already available as a program action (`POST /programs/{name}/install`).

Removed files:
- cli/src/castle_cli/commands/sync.py

Updated files:
- cli/src/castle_cli/main.py (removed sync subparser and dispatch)
- README.md (removed sync from Quick Start and CLI reference)

cli/src/castle_cli/main.py

@@ -59,9 +59,6 @@ def build_parser() -> argparse.ArgumentParser:
     build_parser = subparsers.add_parser("build", help="Build projects")
     build_parser.add_argument("project", nargs="?", help="Project to build (default: all)")
 
-    # castle sync
-    subparsers.add_parser("sync", help="Sync submodules and install dependencies")
-
     # castle gateway
     gateway_parser = subparsers.add_parser("gateway", help="Manage the Caddy gateway")
     gateway_sub = gateway_parser.add_subparsers(dest="gateway_command")
@@ -163,11 +160,6 @@ def main() -> int:
 
         return run_build(args)
 
-    elif args.command == "sync":
-        from castle_cli.commands.sync import run_sync
-
-        return run_sync(args)
-
     elif args.command == "gateway":
         from castle_cli.commands.gateway import run_gateway