feat: add decision helpers and 0.7.4 release prep

[rlippmann]

What changed

• Added public decision helpers to reduce typo-prone direct decision dict traversal in app code:
  • is_update, is_clarify, is_passthrough
  • get_clarify_prompt, get_decision_state
• Exported policy constants at package root:
  • POLICY_USE, POLICY_PROHIBIT
• Updated README and user-facing examples to prefer helper-based decision handling.
• Updated integration examples (LiteLLM and OpenWebUI) to use helper-style decision access instead of raw decision[...] branching.
• Updated integration examples to use POLICY_USE / POLICY_PROHIBIT where policy-value filtering is shown.
• Added focused tests and API conformance fixture coverage for new public exports.
• Centralized canonical decision constants in const.py.
• Removed the legacy context_compiler.decision_constants module.
• Bumped package version from 0.7.3 to 0.7.4.
• Updated OpenWebUI pipe frontmatter metadata:
  • open_webui_pipe.py: version: 0.9.2, requirements: context-compiler>=0.7.4
  • open_webui_pipe_with_preprocessor.py: version: 0.9.2, requirements: context-compiler[experimental]>=0.7.4
• Updated OpenWebUI integration README references from >=0.7.2 to >=0.7.4.

Why

• Improve public API ergonomics before TypeScript 0.7 parity locks.
• Reduce typo-prone direct dict access in normal host code while keeping dict/JSON wire contracts explicit and unchanged.
• Keep Python as the clean source contract so TS can align to it directly.
• Ensure integration copy-paste paths match the recommended helper style and current package version.

Checklist

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