Add initial CBE project docs: AGENTS.md, architecture, and roadmap

[alethkit]

Motivation

• Introduce project-level guidance for the CBE fork derived from QBE to capture development conventions and constraints.
• Record the intended transformation pipeline and artifact model to guide future implementation and design decisions.
• Provide a concise roadmap of milestones from preserving the baseline to provenance and invalidation to align priorities.

Description

• Add AGENTS.md containing CBE contributor instructions including preservation of QBE behavior, keeping master as the upstream baseline, working on cbe (or work in Codex Cloud), and requirements for tests or manual validation for behavior-changing patches.
• Add docs/architecture.md that outlines the planned transformation and artifact pipeline from QBE IL through SSA/block-argument normalization to olog-shaped artifacts, derived facts, proof obligations, and provenance.
• Add docs/roadmap.md listing milestones M0–M5 covering baseline preservation, machine-readable IR dump, phi-to-block-argument normalization, olog JSON export, structural proof obligations, and dependency provenance/invalidation.

Testing

• No automated tests were added or executed as part of this documentation-only change.

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Codex Task