feat(tui): Editor specific settings on tui.json

[trestini]

Issue for this PR

Closes #32329

Type of change

• New feature

What does this PR do?

Add two optional editor settings to tui.json — editor_path and editor_temp_dir — exclusively for the TUI (not the desktop app).

What this changes:

• editor_path — Lets users override the editor command instead of relying solely on VISUAL/EDITOR environment variables. This is useful when you want a different editor inside opencode TUI than your system default.
• editor_temp_dir — Lets users choose where temporary editor files (the .md files created when you press Ctrl+E to edit a prompt) are written. Currently these always go to the OS temp directory (os.tmpdir()).

How changes was done:

• a new 'editor' was added to tui.json schema with above configuration
• created a new EditorConfig type to reflect the schema
• used the newly created object to send parameters to openEditor() function, preserving the default values when they aren't configured.

How did you verify your code works?

• ran bun turbo typecheck
• made sure that no linter problems was added
• I used opencode for some days with this change in order to check if there's a side effect

Screenshots / recordings

Not an UI change

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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