docs: tighten README authority framing

[rlippmann]

What changed

• tightened the README so it leads with Context Compiler as the authority-layer package for deterministic conversational state transitions
• reduced prompt-reinjection framing in the README and kept that comparison in a smaller FAQ role
• reordered the README so quickstart comes before results and FAQ comes after the higher-level architecture/examples material
• updated architecture and milestone docs to match the clean-break ownership split across core, directive drafter, and example integrations

Why

• the README still gave too much weight to prompt reinjection instead of leading with Context Compiler itself
• the supporting docs still needed ownership and roadmap wording that matched the post-clean-break repository boundaries

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)