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.

[kvandre12-commits]

Updated this branch to current main and revalidated the config-cache behavior against the intervening configuration changes.

During validation, the external-edit regression test was found to be writing literal \\n characters rather than valid INI newlines. I corrected the fixture and kept the production parser behavior unchanged.

Validation on Python 3.14:

• 296 passed across tests/test_config_full_coverage.py, tests/agents/test_event_stream_handler.py, and tests/messaging/test_rich_renderer.py
• Ruff E4,E7,E9,F,I checks passed for all changed files
• Ruff formatting check passed
• git diff --check passed

The branch now includes current upstream main without a history rewrite.