Respect model option for agent create

[GreyforgeLabs]

Summary

• Make agent create --model an explicit optional override instead of an always-present default that only generation used.
• Write the requested model into normal created agent frontmatter in non-interactive mode and use it as the interactive model prompt default.
• Keep gpt-4o as the default generation model when --generate is used without --model.
• Add binary CLI coverage for agent create --model and for preserving no model override when omitted.

Fixes PlatformNetwork/bounty-challenge#52744.

Validation

• cargo test -p cortex-cli --test agent_create_model --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check
• git diff --cached --check

[greptile-apps]

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