Cache config reads on streaming and rendering hot paths

[kvandre12-commits]

Summary

Fixes #784.

code_puppy/config.py was re-reading and INI-parsing puppy.cfg on repeated
configuration lookups. Those getters are used from streaming and rendering
hot paths, turning ordinary display decisions into repeated filesystem I/O
and config parsing.

This PR addresses the issue at the three affected boundaries:

• config.py — caches the parsed ConfigParser using the config file's
  (st_mtime_ns, st_size) signature. In-process mutations explicitly
  invalidate the cache, while external file changes are still detected.

• agents/event_stream_handler.py — snapshots the output level and
  suppression decisions once when handling a stream instead of resolving
  them repeatedly as streamed deltas arrive.

• messaging/rich_renderer.py — resolves output_level once for a
  message dispatch and reuses that value through collapse, suppression,
  and subagent rendering decisions.

The result keeps the existing configuration semantics while removing repeated
disk reads and INI parsing from high-frequency streaming and rendering paths.