Feature Request: Support Headless Server and Single-Command Execution

## Context

Exxperts is currently primarily designed for interactive use. To enable integration with CI/CD pipelines, automated workflows, and background processes, Exxperts should support headless operation similar to OpenCode.

## Proposed Solution
Add two execution modes to the Exxperts CLI:
- serve: Starts Exxperts as a long-running, headless server that can be accessed remotely.
- run: Executes a single command or prompt non-interactively and exits once the task is complete.
This would make it possible to build agent loops with Exxperts that run in pipelines or as background processes without requiring an interactive terminal.

## Use Cases
- Run an Exxpert as a step in a GitHub Actions or GitLab CI pipeline.
- Host Exxperts on a server, VPS, or Kubernetes cluster.
- Run autonomous agent loops in the background.
- Execute a single task from a shell script or another application.

## Acceptance Criteria
- The CLI provides a serve command for starting a headless Exxperts server.
- The CLI provides a run command for executing a single prompt or command.
- run exits after the task has completed.
- run returns an appropriate exit code for success and failure.
- Both modes work in environments without a TTY, such as Docker containers and CI runners.
- Server configuration, including host, port, and authentication, can be configured through environment variables or a configuration file.

Begin at the TypeScript CLI entry point and inspect existing command, configuration, and process-exit tests; the issue names no specific files. Trace how interactive execution starts, then map the serve/run lifecycle and non-TTY configuration requirements. Done means both commands work in CI or containers, run exits with meaningful success or failure codes, and server settings support environment variables or a configuration file.

Written by the indexing model from the issue text.