Complete copilot chat integration with lua-tiktoken support

[Copilot]

This PR finishes the copilot installation in plugins/optional/copilot.nix by adding comprehensive chat integration with CopilotChat.nvim and lua-tiktoken support as requested.

Changes Made

Fixed CopilotChat.nvim Setup

The existing configuration had an incorrect require statement and missing setup call:

-- Before: incomplete setup
require('copilotChat')

-- After: proper configuration
require('CopilotChat').setup({
  -- Comprehensive chat settings with window layout, mappings, etc.
})

Added lua-tiktoken Integration

Integrated @gptlang/lua-tiktoken as specified in the requirements:

• Added as flake input in flake.nix
• Built as vim plugin using pkgs.vimUtils.buildVimPlugin
• Included in extraPlugins for enhanced token counting in chat contexts

Enhanced Chat Features

• Window configuration: Floating layout with cursor-relative positioning
• Comprehensive mappings: Tab completion, submit prompts, diff handling
• Useful keymaps: Leader key bindings for common chat operations:
  • <leader>cc - Open Copilot Chat (normal/visual mode)
  • <leader>ce - Explain code
  • <leader>ct - Generate tests
  • <leader>cr - Review code
  • <leader>cf - Fix diagnostic issues
  • <leader>co - Optimize code

Module Integration

Updated full.nix to properly pass inputs parameter to the copilot module, ensuring lua-tiktoken can be built from the flake input.

Testing

Created comprehensive validation tests that verify:

• Correct require statements and setup calls
• Proper lua-tiktoken integration
• All configuration elements are present and syntactically correct

All tests pass, confirming the implementation meets the requirements for finished copilot installation with chat integration and lua-tiktoken support.

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