docs: simplify README for users

[Deicyde]

Depends on #96, which repairs the Setup entrypoints this README advertises. Merge #96 first.

Summary

• rewrite the root README around installation and the everyday Setup → Roadmap → Formalize → Review workflow
• use the installed-plugin Codex names $autoform:setup, $autoform:roadmap, $autoform:formalize, $autoform:human-review, and $autoform:agent-review
• retain the supported-branch/deprecated-execution warning and the local-vs-public publication boundary
• keep compact links to the blueprint/CLI reference, executable example, server operations, and contributor commands
• reduce the README from 153 to 83 lines without duplicating the technical references

Official Codex plugin-skill invocation example: https://learn.chatgpt.com/docs/security/plugin/code-changes#automate-reviews-in-cicd

Validation

• tests/test_skill_examples.py: 22 passed
• git diff --check: clean
• GitHub Python, Windows, real-Lean, and CLA checks run on the integrated current-main head

[Deicyde]

Exact-head review is complete at 0c84441b. The README remains compact (83 lines) while restoring the supported-branch warning, agent-facing Quick start contract, publication boundary, blueprint/CLI/example/server links, and contributor checkout. Codex invocations now use the documented installed-plugin $autoform:skill namespace. Both GitHub Python 3.10/3.13, Windows, real-Lean, and CLA runs are green. No README blocker remains; the blocked label is merge order only—merge #96 first so Setup no longer references deleted helpers.