docs: add agent architecture development roadmap

[arthurpanhku]

What changed

• Added an Agent Architecture Development Roadmap to README.md.
• Organized the work into M0–M4 milestones covering repository governance, task contracts and plan–act–evaluate orchestration, durable execution, observability/tool governance, and evaluation/release gates.
• Added explicit exit criteria and a shared Definition of Done so roadmap items can be converted into focused issues and pull requests.
• Corrected the current LangGraph capability description so durable checkpointing and resumable execution are identified as planned work rather than completed functionality.

Why

The roadmap translates Microsoft Learn's agent architecture and SDLC integration guidance into concrete DocSentinel development work. It gives maintainers a prioritized sequence for improving safety, reliability, auditability, and quality governance.

Impact

Documentation only. No runtime behavior, APIs, schemas, dependencies, or generated contracts are changed.

Validation

• git diff --check
• Confirmed the diff is limited to README.md
• Reviewed milestone headings, checklist structure, and links