refactor: rename internal state terminology

[rlippmann]

What changed

• Rename the private Python _State implementation type to _WorkingMemory throughout the engine.
• Introduce lowercase “working memory” as descriptive terminology in the README opening and State Model guidance.
• Align replacement examples with the working-memory terminology.

Why

The internal name better describes the premise and policy data held by the Engine while remaining private and leaving the public API unchanged.

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 (0a4b0e7) to head (ddc0735).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #277   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          773       773           
=========================================
  Hits           773       773           

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