[codex] Add shared test fixture factories

[JRS1986]

Closes #8.

Agentic coding disclosure

• Agent / tool used: Codex
• Model / provider: OpenAI GPT-5 class model via Codex
• Human operator: @JRS1986
• Task: Add shared pytest fixture factories for repeated RoutingPlan, Provider, HardwareProfile, and scaffold input setup.

What changed

• Files changed:
  • tests/conftest.py adds shared fixture factories and a ScaffoldInputs bundle.
  • tests/test_model_selection.py, tests/test_router.py, tests/test_writers.py, and tests/test_updater.py now use the shared fixtures where they previously repeated constructors.
• Commands run:
  • uv run ruff check
  • uv run pytest -q
  • uv run ruff format tests/conftest.py tests/test_router.py tests/test_model_selection.py tests/test_writers.py tests/test_updater.py
  • uv run coding-scaffold context lint --target .
  • uv run coding-scaffold eval run --target .
• Tests run: 330 passed in 9.90s; ruff passed; context lint found 0 findings; eval passed 9/9 checks.
• Tests not run and why: n/a; all repo-required checks were run.

Risk surface

• External tools or MCP servers used: GitHub plugin/CLI for issue, branch, PR, and merge workflow; no project runtime tooling added.
• Secrets / data exposure risk: none; no credential files or private data inspected.
• Network calls made during the session: GitHub issue/PR/auth/branch operations only.

Review focus

• Human review focus:
  • Confirm fixture defaults preserve the original test scenarios.
  • Confirm the ScaffoldInputs bundle helps readability without hiding behavior-critical setup.
• Known limitations: The local session trace is .coding-scaffold/sessions/2026-05-19-agentic-change.md; this repo ignores session traces by default, so it is referenced here rather than committed.
• Reusable knowledge captured: Shared pytest factory fixtures are a good default for scaffold dataclasses reused across multiple test modules.