Add `agentseek doctor` diagnostics command

Summary

Add an agentseek doctor command that checks the local project environment and reports common setup problems before users run the project.

Requirements

• Add a top-level agentseek doctor command.
• Check whether the current directory has an AgentSeek-style project layout, including .env and pyproject.toml when applicable.
• Check whether required local tools are available, at minimum uv; check docker when a compose file is present.
• Report the detected run mode when it can be inferred.
• Print human-readable pass/warn/fail results and exit non-zero when required checks fail.
• Do not start services, install dependencies, or modify files.

Acceptance Criteria

• Unit tests cover a healthy project, missing .env, missing uv, and compose project without Docker.
• Diagnostics are deterministic and do not depend on actually launching the project.