XY-1162: Add source-backed memory knowledge roadmap

[yvette-carlisle]

Summary

• add the retained source-backed memory + knowledge roadmap under docs/reference/plans
• align README positioning with the source-backed project memory contract
• update the plan index and docs log

Validation

• cargo make check-docs

Claim boundaries

• this is a retained planning artifact; docs/spec/agent_memory_knowledge_system_v1.md remains normative
• no broad competitor superiority, hosted parity, private/provider-backed quality, or small-model authority claim is made

[yvette-carlisle]

R0 follow-up pushed to strengthen docs/OKF readiness. Local validation passed: cargo make check-docs, decodex docs check (concepts=132 links=0 root=docs status=pass), cargo make fmt-rust-check, cargo make check-rust, and cargo make external-memory-radar-dry-run. Fresh read-only skeptic review reported no P0/P1/P2 and no commit blocker. CI is running for head 4c928b249ae54c192469941633c87293c3825c0c.