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`.

doc/diffs.nvim.txt

@@ -41,6 +41,7 @@ CONTENTS                                                      *diffs-contents*
        Fugitive .......................................... |diffs-fugitive|
        Neogit .............................................. |diffs-neogit|
        Gitsigns .......................................... |diffs-gitsigns|
+       Telescope ........................................ |diffs-telescope|
   8. Conflict Resolution .................................... |diffs-conflict|
   9. Merge Diff Resolution ..................................... |diffs-merge|
  10. API ......................................................... |diffs-api|
@@ -84,6 +85,7 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
       neogit = false,
       gitsigns = false,
       committia = false,
+      telescope = false,
       extra_filetypes = {},
       highlights = {
         background = true,
@@ -187,6 +189,16 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
                                  vim.g.diffs = { committia = true }
 <
 
+        {telescope}          (boolean|table, default: false)
+                             Enable telescope.nvim preview highlighting.
+                             Pass `true` or `{}` to enable, `false` to
+                             disable. When active, telescope preview
+                             buffers showing diffs receive treesitter
+                             syntax, line backgrounds, and intra-line
+                             diffs. See |diffs-telescope|. >lua
+                                 vim.g.diffs = { telescope = true }
+<
+
         {extra_filetypes}    (table, default: {})
                              Additional filetypes to attach to, beyond the
                              built-in `git`, `gitcommit`, and any enabled
@@ -577,6 +589,28 @@ backgrounds, and intra-line diffs.
 Highlights are applied in a separate `diffs-gitsigns` namespace and do not
 interfere with the main decoration provider used for diff buffers.
 
+------------------------------------------------------------------------------
+TELESCOPE                                                  *diffs-telescope*
+
+Enable telescope.nvim (https://github.com/nvim-telescope/telescope.nvim)
+preview highlighting: >lua
+    vim.g.diffs = { telescope = true }
+<
+
+Telescope does not set `filetype=diff` on preview buffers — it calls
+`vim.treesitter.start(bufnr, "diff")` directly, so diffs.nvim's `FileType`
+autocmd never fires. This integration listens for the
+`User TelescopePreviewerLoaded` event and attaches to the preview buffer.
+
+Pickers that show diff content (e.g. `git_bcommits`, `git_status`) will
+receive treesitter syntax, line backgrounds, and intra-line diffs in the
+preview pane.
+
+Known issue: Telescope's previewer may render the first line of the preview
+buffer with a black background regardless of colorscheme. This is a
+Telescope artifact unrelated to diffs.nvim. Tracked upstream:
+https://github.com/nvim-telescope/telescope.nvim/issues/3626
+
 ==============================================================================
 CONFLICT RESOLUTION                                          *diffs-conflict*
 

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()

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)