Add custom statusline module and remove lualine config

[halkn]

Motivation

• Replace the external lualine config with a lightweight, self-contained Lua statusline to reduce dependency and provide tighter integration with local utilities.
• Provide combined information including mode, git branch, diagnostics, file flags, file metadata, and LSP/progress status in a single renderer.

Description

• Add lua/modules/statusline.lua which implements a custom statusline renderer with mode name mapping, diagnostics_summary, git_branch (with per-repo caching), progress_summary, and render/setup functions.
• Use vim.fs.root/vim.fs.find to locate git roots and fall back to gitsigns_head when available, caching branch names per repository and clearing the cache on DirChanged via an autocmd group.
• Wire the statusline into Neovim by setting vim.o.laststatus = 3 and vim.o.statusline = '%!v:lua.dotfiles_statusline_render()', and expose the renderer as _G.dotfiles_statusline_render.
• Remove the previous lua/plugins/lualine.lua configuration file to stop using nvim-lualine/lualine.nvim.

Testing

• No automated tests were run for this change.

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Codex Task