Neovim migration — Phase 4: Replace syntastic with built-in LSP + nvim-lint

[mattmenefee]

Blocked by

• Neovim migration — Phase 3: Replace Vundle with lazy.nvim #69

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Replace synchronous syntastic with Neovim's async-native diagnostics stack: built-in LSP + nvim-lint for non-LSP linters.

Depends on: Phase 3 (lazy.nvim required).

Why

• syntastic blocks the UI on save (shells out and waits synchronously)
• Neovim has vim.diagnostic and vim.lsp built in since 0.5
• LSP also provides go-to-definition, hover docs, rename, code actions — free upside

Changes

• Add neovim/nvim-lspconfig and williamboman/mason.nvim for managing LSP servers
• Add mfussenegger/nvim-lint for non-LSP linters
• Replace airblade/vim-gitgutter with lewis6991/gitsigns.nvim (also async)
• Remove Plugin 'scrooloose/syntastic' and Plugin 'myint/syntastic-extras'
• Remove all let g:syntastic_* config blocks

LSP servers to configure (matched to current stack)

• Ruby: ruby_lsp or solargraph
• JS/TS: ts_ls
• Terraform: terraformls
• YAML: yamlls
• Bash: bashls

Linters to keep via nvim-lint

• scss_lint
• haml_lint
• pyyaml
• language_check (gitcommit)

Acceptance

• Save no longer pauses the UI
• Diagnostic keymaps (next/prev error, hover, code action) configured
• :LspInfo shows attached servers for each language in our stack
• gitgutter-style signs still appear in the sign column