feat(models): add safe description overlays for model metadata + surface descriptions in model picker/settings

[B-Deforce]

Summary

What this changes

• Adds code_puppy/model_descriptions.py with:

  • apply_description_overlays(...)
  • get_model_description(...)
  • default fallback text

• Adds a new callback hook:

  • load_model_descriptions in code_puppy/callbacks.py

• Updates ModelFactory.load_config() to apply description overlays after config merges:

  • bundled models.json descriptions
  • plugin-provided description overlays
  • only fills missing/blank descriptions (no clobbering of model settings)

• Improves model UX:

  • /model completion now shows description metadata (with fallback)
  • model settings menu shows selected model description (with fallback)

• Updates code_puppy/models.json descriptions for current OSS models.

• Adds/updates tests:

  • new tests/test_model_descriptions.py
  • expanded picker + settings coverage for description/fallback behavior
  • updates docs hook table in AGENTS.md for load_model_descriptions

Why

Model configs are merged from multiple sources using shallow updates.
Without a description-only overlay pass, metadata can be lost or overwritten unintentionally.
This keeps descriptions reliable while preserving all non-description model fields.

Guardrails

apply_description_overlays(...) intentionally:

• does not create new models
• does not overwrite non-description fields
• only sets description when missing/blank
• ignores invalid/empty overlay values

Validation

Ran formatting/lint on touched files:

• isort --profile black ...
• ruff format ...
• ruff check --fix ...
• ruff check ...

Ran targeted tests:

• tests/test_model_descriptions.py
• tests/command_line/test_model_picker_completion.py
• tests/command_line/test_model_settings_menu_coverage.py

All passed locally.