Add `--output-dir` to `agentseek create`

Summary

Add an --output-dir option to agentseek create so users can choose where the generated project is written.

Requirements

• Support agentseek create <spec> --output-dir <path>.
• Default behavior should remain unchanged: generated projects are written under the current working directory.
• Pass the selected path to Cookiecutter as its output_dir.
• The option should work for bundled templates and external Cookiecutter templates.
• Preserve existing --no-input, --template, and --checkout behavior.

Acceptance Criteria

• Unit tests cover default current-directory output and explicit output directory output.
• Tests cover both bundled template resolution and external template passthrough.
• Existing agentseek create tests continue to pass.