docs: split README into a docs/ tree and fill developer + maintainer depth

[hyperpolymath]

A documentation survey during the extraction close-out found the conceptual docs are strong (ARCHITECTURE, EXPLAINME, DISAMBIGUATION, GOVERNANCE, MAINTAINERS, policies/README) but nothing ties them together, the README is overloaded (~190 lines doing the job of docs/), and per-component developer depth is missing. docs/tech-debt-2026-05-26.adoc already recommends the split.

Structure / navigation

• Add a docs/ index (docs/README.adoc) — a documentation map by audience (developer / maintainer / end-user) linking the existing docs. (quick win — see close-out)
• Split the overloaded README.adoc into a thin entry point + docs/architecture.adoc, docs/usage.adoc, docs/policies.adoc.

Developer depth

• Per-crate READMEs: oikosbot-cli, -analysis, -metrics, -sarif, -eclexia (only oikosbot-fleet has one today) — purpose, how it fits, key types.
• analyzers/code-haskell/README — no docs today; build (cabal build/test all), the Eco.*/Quality.* modules, and the heuristic (not ontology-driven) nature.
• End-to-end build walkthrough across Rust + Haskell + AffineScript (currently scattered across QUICKSTART/PLAYBOOK).
• Expand the CONTRIBUTING.md stub (20 lines): code style, commit convention, the a2ml/STATE session workflow.

Maintainer depth

• CI/workflow runbook — what each .github/workflow does (the governance battery, hypatia-scan, scorecard, secret-scanner, codeql, language-policy).
• just target reference (what just check / maint-audit / release-preflight actually run).
• Pre-release checklist + gitbot-fleet coordination notes.

Surfaced during the post-extraction close-out.