[Automation] Introduce noxfile.py to consolidate dev tooling and CI tasks

[williaby]

Summary

Introduce a centralized noxfile.py to manage dev and CI automation tasks in a Python-native, composable format. This will reduce redundancy across CI workflows and provide a uniform interface for local development and testing.

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Design Goals

• Define reusable sessions for:

  • lint: black, isort, flake8
  • typecheck: mypy
  • security: bandit, safety, pip-audit
  • secrets: invoke TruffleHog (or shell out as needed)
  • tests: pytest
  • sbom: optional CycloneDX or Trivy wrapper

• Replace workflow boilerplate by executing:

  • nox -s lint
  • nox -s tests
  • nox -s security

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Implementation Tasks

• Create noxfile.py in project root
• Define sessions: lint, typecheck, security, tests, sbom, format
• Install session dependencies using Poetry or pip-tools
• Update local docs: replace make lint or pytest with nox
• Refactor CI jobs to use nox commands in place of raw pip install and repeated run: ... blocks

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Acceptance Criteria

• All existing CI logic runs through nox
• Local development mirrors CI with one-command tasks
• Each workflow runs at or below current runtime thresholds
• Redundant tool declarations in workflows are removed
• All tool configs are centralized in pyproject.toml

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Dependencies

• Poetry migration (#XYZ)
• Consolidate setup.cfg into pyproject.toml (#XYZ)
• Pre-commit and dev tooling already defined via Evaluate Long-Term Backup Strategy for LedgerBase Deployments #93

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Milestone

Milestone: v0.4.0 – Unified Developer Tooling

Nox will form the automation backbone of the development lifecycle, enabling reproducible local workflows and simplifying CI/CD orchestration.