Add Session Indicators with Customizable Prompt Styles

[K-NRS]

Summary

This PR adds visual session indicators to help users know when they're inside an NDS session. The implementation is completely automatic and non-invasive.

Features Added

• Terminal Title: Shows NDS: session-name in terminal window title
• Prompt Indicators: Configurable visual prefix for shell prompts
• Multiple Styles: Support for different indicator styles via NDS_PROMPT_STYLE
• Shell Agnostic: Works with bash, zsh, and other common shells
• Zero Configuration: Works out of the box, no user config modification required

Indicator Styles

Users can control the appearance with the NDS_PROMPT_STYLE environment variable:

• subtle (default): ⬢ symbol prefix
• full: [nds:session-name] prefix
• minimal: [nds] prefix
• none: completely disabled

Implementation Details

• Automatic setup during session spawn and attach
• Shell-specific initialization scripts for optimal compatibility
• Environment variables set for user customization
• No modification of user's shell configuration files

Testing

• ✅ All existing tests pass
• ✅ Code compiles without warnings
• ✅ Rustfmt formatting enforced
• ✅ Backward compatibility maintained

Test Plan

• Test with different shells (bash, zsh, fish)
• Verify all indicator styles work correctly
• Confirm terminal title updates
• Test session creation and attachment
• Verify no impact on existing functionality

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