fix(startup): invoke MCP installation as a Python module

[LinXunFeng]

Summary

Running ./start.sh with uv 0.5.9 from the repository root can fail during MCP installation because uv executes the local artemis directory. This allows artemis/platform to shadow the standard-library module and causes a langsmith.RunTree import error.

Use uv run python -m artemis for MCP installation and its printed retry instructions, matching the existing UI launch command.

Closes #106

Validation

Passed:

• bash -n start.sh
• bash start.sh --help
• uv run python -m artemis mcp --help
• uv run python -m artemis mcp --install all
• git diff --check
• make typecheck
• uv run python scripts/quality_ratchet.py

Additional repository checks:

• make test: 2108 passed, 81 failed, 6 skipped, 8 deselected.
  Failures include missing Gemini credentials and assertions outside the changed startup script.
• make lint: formatting passed; Ruff reported 11 existing violations in playground/backend_manager.

Only start.sh is changed. No dependency changes.