fix: honor configured graph model in cognify

[mikemikimike]

Fixes #4632

Root cause: config.set_graph_model() stored the model in GraphConfig, but cognify() and get_default_tasks() defaulted directly to KnowledgeGraph and never read it.

The fix resolves the configured model at call time when graph_model is omitted, preserves explicit graph_model precedence, and adds regression coverage at both the cognify forwarding layer and direct get_default_tasks() task construction layer.

Validation:

• python -m py_compile passed for changed Python files
• ruff check passed when available in the workspace
• git diff --check passed
• Full repository pytest was attempted but could not complete because upstream source downloads were repeatedly truncated by the network environment.