feat: add verified maintenance autopilot foundations

[kam6l]

Summary

• Maintenance Foundations: Added maintenance graph models, priority scoring, dependency scheduling, and local SQLite state store.
• Proof & Trust Primitives: Added delta/adversarial proof checking, flake firewall, proof leasing, and post-merge outcome verification.
• Delivery & Isolation: Added patch minimizer, PR reconciler, and untrusted generator isolation harness.
• Documentation & Architecture: Added updated engineering documentation for architecture, security invariants, design system, and testing gates.

Validation

• uv run pytest -p no:cacheprovider tests/: 488 passed, 2 skipped
• uv run ruff format --check src tests examples benchmarks demos: clean
• uv run ruff check src tests examples benchmarks demos: clean
• uv run mypy src/agentdiff: 135 files checked, 0 errors