fix: add terminal window validation and configurable persistence

[alvarosevilla95]

Summary

Fixes terminal window validation issues and adds configurable terminal persistence.

Changes Made

1. Window Validation Fix (Snacks Provider)

   • Adds validation to check if the terminal window is still valid before attempting to focus it
   • If the window was closed, reopens the terminal instead of throwing an error
   • Prevents crashes when the terminal window is manually closed and the user tries to toggle focus

2. Configurable Terminal Persistence (Native Provider)

   • Adds preserve_on_close configuration option (default: false)
   • When true, preserves terminal process when window is manually closed (native provider only)
   • Maintains backward compatibility with existing behavior
   • Provides same functionality as snacks provider but makes it configurable

Configuration

Users can now enable terminal persistence by setting:

require('claudecode').setup({
  terminal = {
    preserve_on_close = true, -- Preserve terminal process when window is closed
    -- other terminal options...
  }
})

Default behavior (preserve_on_close = false): Terminal buffer is wiped when window is closed (original behavior)
New behavior (preserve_on_close = true): Terminal buffer is hidden when window is closed, preserving the process for later restoration

Technical Details

• Both providers now handle closed terminal windows gracefully
• Terminal processes are preserved during programmatic hiding (toggle functions) regardless of the setting
• The preserve_on_close setting only affects manual window closing (X button, :q, etc.)
• Comprehensive documentation added to README for the new configuration option