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`.

lua/diffs/init.lua

@@ -42,6 +42,8 @@
 
 ---@class diffs.CommittiaConfig
 
+---@class diffs.TelescopeConfig
+
 ---@class diffs.ConflictKeymaps
 ---@field ours string|false
 ---@field theirs string|false
@@ -68,6 +70,7 @@
 ---@field neogit diffs.NeogitConfig|false
 ---@field gitsigns diffs.GitsignsConfig|false
 ---@field committia diffs.CommittiaConfig|false
+---@field telescope diffs.TelescopeConfig|false
 ---@field conflict diffs.ConflictConfig
 
 ---@class diffs
@@ -149,6 +152,7 @@ local default_config = {
   neogit = false,
   gitsigns = false,
   committia = false,
+  telescope = false,
   conflict = {
     enabled = true,
     disable_diagnostics = true,
@@ -607,6 +611,10 @@ local function init()
     opts.committia = {}
   end
 
+  if opts.telescope == true then
+    opts.telescope = {}
+  end
+
   vim.validate('debug', opts.debug, function(v)
     return v == nil or type(v) == 'boolean' or type(v) == 'string'
   end, 'boolean or string (file path)')
@@ -623,6 +631,9 @@ local function init()
   vim.validate('committia', opts.committia, function(v)
     return v == nil or v == false or type(v) == 'table'
   end, 'table or false')
+  vim.validate('telescope', opts.telescope, function(v)
+    return v == nil or v == false or type(v) == 'table'
+  end, 'table or false')
   vim.validate('extra_filetypes', opts.extra_filetypes, 'table', true)
   vim.validate('highlights', opts.highlights, 'table', true)
 
@@ -1012,6 +1023,12 @@ function M.get_committia_config()
   return config.committia
 end
 
+---@return diffs.TelescopeConfig|false
+function M.get_telescope_config()
+  init()
+  return config.telescope
+end
+
 ---@return diffs.ConflictConfig
 function M.get_conflict_config()
   init()