feat: add PostHog usage events + release automation

[abrichr]

Summary

• add openadapt_telemetry.posthog with privacy-safe usage event capture
• expose capture_usage_event and capture_posthog_event in package API
• add release/test GitHub Actions for semantic-release + trusted publishing to PyPI
• add tests for PostHog usage event behavior and docs for env config

Validation

• ruff check on changed files
• PYTHONPATH=src uv run pytest -q tests/test_posthog.py (4 passed)

Follow-ups

• configure PyPI trusted publisher for openadapt-telemetry
• set ADMIN_TOKEN repo secret for protected-branch semantic-release pushes