refactor(chat): move completion logic to separate module

[deathbeam]

BEGIN_COMMIT_OVERRIDE
feat(completion): add support for omnifunc and move completion logic to separate module

This change refactors the chat completion logic by moving it from
init.lua into a new completion.lua module. It also updates mappings
and setup to use the new module, improving code organization and
maintainability. Autocomplete setup is now handled in the completion
module, and prompt listing is extracted to a helper function.

Together with vim.o.autocomplete and vim.o.complete=".,o" we can achieve autocomplete in normal way as well now with just omnifunc
END_COMMIT_OVERRIDE

[Copilot]

Pull Request Overview

This PR replaces the automatic chat autocompletion mechanism with Vim's omnifunc feature to provide autocompletion in a more standard way. The change eliminates the need for the chat_autocomplete configuration option and associated TextChangedI autocmd complexity.

• Removes the chat_autocomplete configuration option and its associated logic
• Implements a new CopilotChatOmnifunc function to handle autocompletion via Vim's omnifunc
• Simplifies buffer setup by removing autocmd-based completion triggers

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lua/CopilotChat/init.lua	Adds omnifunc implementation and removes TextChangedI autocmd-based autocompletion logic
lua/CopilotChat/config.lua	Removes the chat_autocomplete configuration option

Comments suppressed due to low confidence (1)

lua/CopilotChat/init.lua:775

• [nitpick] The function name should follow Lua naming conventions. Consider renaming to copilot_chat_omnifunc (snake_case) to maintain consistency with other local functions in the codebase.

function CopilotChatOmnifunc(findstart, _)