fix: preserve mode-specific model selections across mode switches

[sudo-tee]

This should fix #112

[Copilot]

Pull request overview

This PR fixes issue #112 where mode switching via Alt+M does not respect model configuration from opencode.json. The fix introduces a user preference tracking system and improves the model selection priority logic.

Changes:

• Added user_mode_model_map state field to track user's model selections per mode
• Modified switch_to_mode to use a priority system: user selection > mode config > global config
• Removed state resets in M.open to preserve mode/model selections across new sessions
• Updated and added tests to validate the new priority-based model selection behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lua/opencode/state.lua	Adds user_mode_model_map field to track user's per-mode model selections
lua/opencode/core.lua	Updates model selection logic in switch_to_mode with priority system, stores user selections in configure_provider, removes state resets in M.open
tests/unit/core_spec.lua	Updates tests to initialize new state field, replaces old test with new tests validating priority-based model selection

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