feat: add run workspace option

[haosenwang1018]

What

Adds gitagent run --workspace <dir> so an agent definition can live separately from the repository or directory it operates on.

Why

Refs #58.

The main use case is running a reusable agent definition, such as a reviewer or autoresearch agent, against a separate target repo without making the agent operate on its own agent.yaml, SOUL.md, and skills directory.

How

• Adds -w, --workspace <dir> to gitagent run.
• Passes the workspace through the normal run path and the -a git delegated path.
• Applies the resolved workspace as the spawned process cwd for direct process runners: Claude, OpenAI, CrewAI, Nanobot, and OpenClaw.
• Documents the behavior and notes that temp-workspace adapters (opencode, gemini, gitclaw) keep using their prepared workspace to avoid overwriting target repo files like AGENTS.md, GEMINI.md, or agent.yaml.

How Tested

• npm run build passes
• npm test passes
• git diff --check passes
• node dist/index.js run --help shows --workspace

Checklist

• My code follows the existing style of this project
• I have added/updated tests (if applicable)
• I have updated documentation (if applicable)
• I have read the CONTRIBUTING.md