Document policy runtime and add enforce-mode example

[gut-puncture]

Summary

This PR finalizes the policy-runtime v2 surface with docs, a runnable example, and small API-surface hardening.

Changes:

• expands concordia/document/README.md with:
  • policy lifecycle
  • observe vs enforce semantics
  • malformed-policy behavior
  • policy tag documentation
  • schema helper usage
• adds deterministic example:
  • examples/tool_policy_example.py
  • demonstrates schema denial + risk-policy denial in enforce mode
• adds constructor validation hardening in InteractiveDocumentWithTools:
  • max_tool_calls_per_question >= 1
  • max_tool_result_length >= 3
  • explicit validation error messages
• adds tests for new constructor validation paths

This branch is independently mergeable from main, so it includes the runtime and schema helper foundations.

Why

This makes the feature reviewer-friendly and production-usable: semantics are clearly documented, example is runnable, and key input contracts are explicit.

Validation

Executed locally:

• . .venv/bin/activate && python -m pytest -q concordia/document/tool_policy_test.py
• . .venv/bin/activate && python -m pytest -q concordia/document/interactive_document_tools_test.py
• . .venv/bin/activate && python -m pytest -q concordia/document
• . .venv/bin/activate && python -m pylint --errors-only on touched files
• . .venv/bin/activate && PYTHONPATH=. python examples/tool_policy_example.py