Add pytest markers for selective test execution

[manavgup]

Context

Borrowed from IBM/mcp-context-forge which uses markers like @pytest.mark.slow, @pytest.mark.e2e, @pytest.mark.integration.

FastStack has 387 tests but no way to run subsets by category. All tests run every time (~5s now, but will grow).

What to do

• Define markers in pyproject.toml: unit, integration, e2e, slow
• Tag existing tests:
  • tests/test_core/ → unit
  • tests/test_templates/ → unit
  • tests/test_cli/ → integration (they scaffold real projects in tmp dirs)
• Add Makefile targets:
  • make test-unit → pytest -m unit
  • make test-integration → pytest -m integration
  • make test-fast → pytest -m "not slow"
• Suppress PytestUnknownMarkWarning

Acceptance criteria

• make test-unit runs only core + template tests
• make test-integration runs only CLI tests
• make test still runs everything

[manavgup]

Closed by PR #16