Add documentation code validation system (#43)

* Add documentation code validation system

- Create runnable example scripts in examples/api/ (19 files)
- Add CodeSnippet component for extracting marked sections
- Add raw-loader dependency for importing source files
- Create sample retain-new.mdx showing new approach
- Add README documenting coverage and gaps

* Fix wheel glob expansion in test-doc-examples CI job

* Fix CI issue

* Fix wheel path - uv build outputs to repo root dist/

* Fix: use explicit shell expansion for wheel install

* Fix: run cd in subshell so install runs from repo root

* Add documentation code validation CI job

- Use uv sync + uv run pattern (matches existing CI)
- Add requests to test dependencies for cleanup scripts

* Fix async API client usage in documents.py example

* Fix main-methods.py: RecallResult and ReflectFact don't have weight attribute

* Fix opinions.py: use actual API attributes instead of non-existent ones

* Fix example scripts: remove non-existent API attributes

- recall.py: remove .weight, fix entities iteration (dict not list)
- retain.mjs: remove result.async check