docs: add proposal 138 — proposals audit, top 10, and 5 novel ideas

[neoneye]

Summary

• Surveys all ~137 existing proposals; classifies by status (implemented / partial / untouched / stale) and theme; lists retirement candidates and already-implemented items so the active set stays scannable.
• Picks the 10 most promising untouched directions and gives each a concrete next-level step (ELO judge, semantic plan graph, routing trio, autonomous prompt optimisation, post-plan agent execution, adversarial premise gate, Fermi sanity gate, ensemble judge + parallel racing, finance triad, structured expert verification).
• Adds 5 novel ideas built on top of that substrate: plan lineage diff, counterfactual forking, replay lab for model-drift, 24h risk curriculum, federated lever library.

Test plan

• Skim docs/proposals/138-proposals.md for clustering accuracy
• Sanity-check the "implemented" and "retirement" lists against your own mental model
• Decide whether to follow the operational recommendation (split folder into done/ and archive/)