Agent becomes one class assembled from mixins: runtime (turn lifecycle),
streaming (the turn loop), tools (tool execution), compression (context
gates) and context (tool lists, backend lookup, window). The module-level
helpers and sentinels move to helpers.py.

ContextBuilder splits into helpers (pure truncation/freshness/prompt
blocks) and builder (the class). Both old modules become packages with
the same import surface, so navi.core.agent.Agent and
navi.core.context_builder.ContextBuilder keep working.

Method bodies are unchanged; tests now patch settings/log/should_compress
on the mixin that actually reads them.