feat: add decision kind constants for host ergonomics

[rlippmann]

What changed

• Added public decision-kind constants in a dedicated module:
  • DECISION_PASSTHROUGH = "passthrough"
  • DECISION_UPDATE = "update"
  • DECISION_CLARIFY = "clarify"
• Exported those constants from package root (context_compiler).
• Added a small value test to assert constant values match the Decision kind strings.
• Updated the public API contract fixture required exports for the new constants.
• Made minimal readability updates in README and one host-facing example to use the constants.
• Follow-up cleanup: replaced raw decision-kind string comparisons with constants across user-facing branching code in examples/, demos/, Open WebUI/LiteLLM integration examples, and REPL rendering logic.

Why

• Reduce repeated semantic string literals in host-facing branching.
• Lower typo risk while preserving the existing structural Decision/State contracts and JSON portability.
• Keep ergonomics improvement intentionally small without adding helper predicates or broader abstractions.

Checklist

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