feat: Allow to extend and customize TUI with additional commands

[shaj13]

Problem

Customizing the TUI currently requires forking the repository or building a UI from scratch. This limits docker-agent's potential to serve as an end-to-end framework where consumers can reuse the TUI while adding their own commands.

What was done

• Added Immediate flag to commands.Item to control whether a command interrupts an ongoing stream
• Refactored ParseSlashCommand to a Parser struct with NewParser constructor, allowing consumers to build parsers with custom command categories
• Added WithCommandBuilder option to allow consumers to provide custom command categories to the TUI

Dependency Resolution

At first, I started with an interface that appModel implements which returns internal objects like *app.App, *service.SessionState, chat.Page, and editor.Editor. However, that interface can't be set in the main package as it may not serve future changes, unless it's set in a dedicated package with a guarantee there will be no circular imports.

So I added a dynamic dependency resolution pattern via core.Resolve[T] - a type-safe generic function that retrieves dependencies from any tea.Model implementing Resolve(any) any. This pattern:

• Avoids circular dependencies between internal packages
• Allows command handlers to access dependencies without tight coupling
• Enables future refactoring of handlers into separate packages

Usage:

app := core.Resolve[*app.App](model)
sessionState := core.Resolve[*service.SessionState](model)
chatPage := core.Resolve[chat.Page](model)

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This may not be the final implementation as I am looking for feedback. I'm not sure if you're willing to allow TUI customization, but I think it would be very useful.