feat: add telescope.nvim integration (#170)

Closes #169.

## Problem

Telescope never sets `filetype=diff` on preview buffers — it calls
`vim.treesitter.start(bufnr, "diff")` directly, so diffs.nvim's
`FileType` autocmd never fires.

## Solution

Add a `telescope` config toggle (same pattern as
neogit/gitsigns/committia) and a `User TelescopePreviewerLoaded` autocmd
that calls `attach()` on the preview buffer. Disabled by default; enable
with `telescope = true`.

Also adds a `diffs-telescope` vimdoc section documenting the integration
and the upstream first-line preview bug
(nvim-telescope/telescope.nvim#3626).

Includes committia.vim integration from `feat/committia`.

plugin/diffs.lua

@@ -18,6 +18,16 @@ if gs_cfg == true or type(gs_cfg) == 'table' then
   end
 end
 
+local tel_cfg = (vim.g.diffs or {}).telescope
+if tel_cfg == true or type(tel_cfg) == 'table' then
+  vim.api.nvim_create_autocmd('User', {
+    pattern = 'TelescopePreviewerLoaded',
+    callback = function()
+      require('diffs').attach(vim.api.nvim_get_current_buf())
+    end,
+  })
+end
+
 vim.api.nvim_create_autocmd('FileType', {
   pattern = require('diffs').compute_filetypes(vim.g.diffs or {}),
   callback = function(args)