Add dry-run mode for `agentseek run`

Summary

Add a --dry-run option to agentseek run so users can inspect how AgentSeek would start a generated project without launching any service or opening a browser.

Requirements

• Support agentseek run --dry-run together with existing options such as --mode, --port, --host, and --no-browser.
• Resolve and print the launch plan:
  • detected or selected launch mode (compose or python)
  • resolved host and port
  • frontend URL
  • command that would be executed
• Do not start Docker Compose, Python entry points, subprocesses, readiness polling, or browser opening in dry-run mode.
• Keep existing non-dry-run behavior unchanged.

Acceptance Criteria

• Unit tests cover compose mode, python mode, explicit port override, and no subprocess execution in dry-run mode.
• uv run python -m pytest tests/cli_commands/test_run.py passes.