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

AGENTS.md

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+# CBE Agent Instructions
+
+- CBE is derived from QBE.
+- The project goal is categorical/proof-artifact infrastructure, not QBE's original minimal-backend goal.
+- Preserve inherited QBE behavior unless a task explicitly says otherwise.
+- Keep `master` as upstream/QBE baseline.
+- Do CBE work on `cbe`; in Codex Cloud, the checked-out task branch may be called `work`, and that is acceptable.
+- Prefer small, reviewable commits.
+- Every behavior-changing patch must include either a test or a clear manual validation command.
+- Do not introduce external dependencies without justification.
+- Keep C code style close to existing QBE style unless a new subsystem clearly needs a different style.
+- New semantic/provenance code should be isolated under clearly named files rather than tangled into backend code.

docs/architecture.md

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+# CBE Architecture (Planned)
+
+Intended transformation and artifact pipeline:
+
+QBE IL
+→ parsed QBE AST / internal structures
+→ block-argument SSA normalization
+→ olog-shaped artifact graph
+→ derived facts
+→ proof obligations
+→ provenance / invalidation

docs/roadmap.md

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+# CBE Roadmap
+
+- M0: preserve baseline build
+- M1: machine-readable IR dump
+- M2: phi-to-block-argument normalization
+- M3: olog JSON export
+- M4: structural proof obligations
+- M5: dependency provenance and invalidation