feat: add automatic context compaction

[QueryPlanner]

What

Add native Google ADK token-based event compaction at 200,000 tokens so long conversations continue in the same session without requiring /reset.

Why

Prevent long-running conversations from growing beyond the model context window while preserving recent context and session continuity.

How

• Configure EventsCompactionConfig with a 200,000-token threshold.
• Retain the latest eight raw events and persist ADK compaction events.
• Add integration and SQLite-backed compaction persistence coverage.
• Document compaction behavior and the continued /reset escape hatch.

Tests

• uv run ruff format --check
• uv run ruff check --output-format=github
• uv run mypy .
• uv run pytest --cov=src --cov-report=xml --cov-report=term-missing (1612 passed, 1 skipped; 100% coverage)
• uv run --no-sync mkdocs build --strict
• bash -n setup.sh entrypoint.sh
• Docker Compose production and development configuration checks
• Docker image build, production smoke test, and OTLP validation; Docker daemon was unavailable locally

Related Issues

• None.

Breaking Changes

None.