feat(tui): display count of modified files in session

[ErvisTusha]

This feature adds a count display for files modified during the current session in the TUI sidebar under the "Modified Files" section. Changes

• packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx: Updated the display logic to show the count of session-modified files, making the expandable UI available when there are multiple files. Technical Details
• Leverages existing session_diff data to count modified files
• Maintains the expandable UI for better user experience with multiple modifications
• Integrates seamlessly with the existing sidebar layout This provides users with immediate visibility into how many files have been modified in their current session.