refactor: centralize llm defaults

[fmueller]

Summary

• set the shared LLM defaults in llm.py, switching the default model to ministral-3:8b and centralizing the timeout
• update idea, brief, meta, writer, and translate flows to consume the shared defaults instead of command-specific constants
• refresh CLI defaults and post-edit settings to reflect the new model name

Testing

• uv run ruff check --fix
• UV_NO_SYNC=1 uv run mypy
• UV_NO_SYNC=1 uv run pytest

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Codex Task