Add quickfix listing and improve conflict handling

[Faria22]

• Expose conflict discovery via M._get_conflicts and reuse it to populate the quickfix list
• Add :HeadhunterQuickfix plus default <leader>gq binding and extend the test suite for parser/quickfix behaviour
• Check against duplicate git-ls entries

[Faria22]

Fixes #5

[Copilot]

Pull Request Overview

This PR adds quickfix list functionality to the headhunter plugin and improves conflict marker handling to support different git conflict styles (HEAD vs stash markers). The changes make conflict discovery reusable and provide a comprehensive view of all conflicts in the quickfix list.

Key changes:

• Expose conflict discovery via M._get_conflicts and add quickfix population functionality
• Generalize conflict marker parsing to handle both HEAD and stash-style conflicts
• Add comprehensive test coverage for the new quickfix integration and stash-style conflict handling

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lua/headhunter/init.lua	Main implementation adding quickfix functionality, exposing _get_conflicts, generalizing conflict parsing, and adding duplicate file handling
lua/tests/quickfix_spec.lua	New test suite for quickfix integration functionality
lua/tests/headhunter_spec.lua	Extended tests for stash-style conflict marker parsing
lua/tests/conflict_resolution_spec.lua	Additional tests for handling stash-style conflict resolution
README.md	Updated documentation to use <cmd> syntax for key mappings

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