docs: streamline 0.9 documentation

[rlippmann]

What changed

• Streamlined the primary README and moved detailed CLI/REPL guidance into dedicated documentation.
• Cleaned up and aligned the API, architecture, milestone, multi-engine, demo, and design-philosophy documentation with the current 0.9 surface.
• Updated the REPL banner to consume the package version string and adjusted its focused tests.

Why

• Remove stale controller, preview, checkpoint, and confirmation-era wording from current documentation.
• Make the public API and architecture easier to scan while preserving authoritative specifications and conformance contracts.
• Keep current and historical behavior clearly distinguished for release readiness.

Checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (38a6365) to head (25c6a18).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #267   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          783       783           
=========================================
  Hits           783       783           

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