[idea] Conform format-on-save: BufWritePre autocmd with per-buffer opt-out toggle

[stanfish06]

What

Add an optional auto-format-on-save behaviour driven by a BufWritePre autocmd that calls conform.format(). Pair it with a buffer-local toggle (<leader>lF) so it can be disabled per-buffer when format noise is unwanted (e.g. exploring a poorly-formatted third-party file).

Where

lua/config/plugin_config.lua — inside the if conform_ok and not is_vscode then block, alongside the existing <leader>lf keymap (lines 152–163).

Why it matters

The config already has <leader>lf for manual format. Auto-format-on-save is the natural next step: it eliminates the manual step for files where a formatter is configured and produces clean commits without requiring discipline. The per-buffer toggle handles the cases where you don't want it (reading upstream code, pairing sessions, etc.).

Suggested implementation

-- inside the conform_ok block:
local auto_format_enabled = true  -- global default

vim.keymap.set("n", "<leader>lF", function()
    vim.b.auto_format = not vim.b.auto_format  -- nil → false (disable)
    local state = vim.b.auto_format == false and "OFF" or "ON"
    vim.notify("Auto-format: " .. state, vim.log.levels.INFO)
end, { desc = "Toggle auto-format for buffer" })

vim.api.nvim_create_autocmd("BufWritePre", {
    callback = function()
        -- skip if explicitly disabled for this buffer
        if vim.b.auto_format == false then return end
        if not auto_format_enabled then return end
        conform.format({ timeout_ms = 500, lsp_format = "fallback" })
    end,
})

Notes

• The timeout_ms cap prevents slow formatters from blocking the write.
• lsp_format = "fallback" means the LSP formatter fires when no conform formatter is configured for the filetype, consistent with the existing default_format_opts in the conform setup.
• A global toggle (auto_format_enabled) can be added later if needed; the per-buffer flag covers the common case.