refactor: standardize log prefixes to [diffs.nvim]:

Problem: `lib.lua` used `[diffs]` and `debug.lua`/`init.lua` used
`[diffs.nvim]` (no colon) while the rest of the codebase used
`[diffs.nvim]:`.

Solution: Normalize all `vim.notify` prefixes to `[diffs.nvim]:` for
consistency with `log.lua` and the other modules.

lua/diffs/debug.lua

@@ -63,7 +63,7 @@ function M.dump()
   if f then
     f:write(vim.json.encode(result))
     f:close()
-    vim.notify('[diffs.nvim] debug dump: ' .. path, vim.log.levels.INFO)
+    vim.notify('[diffs.nvim]: debug dump: ' .. path, vim.log.levels.INFO)
   end
 end
 

lua/diffs/init.lua

@@ -621,7 +621,7 @@ local function migrate_integrations(opts)
       local old = 'vim.g.diffs.{' .. table.concat(stale, ', ') .. '}'
       local new = 'vim.g.diffs.integrations.{' .. table.concat(stale, ', ') .. '}'
       vim.notify(
-        '[diffs.nvim] ignoring ' .. old .. '; move to ' .. new .. ' or remove',
+        '[diffs.nvim]: ignoring ' .. old .. '; move to ' .. new .. ' or remove',
         vim.log.levels.WARN
       )
     end

lua/diffs/lib.lua

@@ -189,7 +189,7 @@ function M.ensure(callback)
   )
 
   local dest = lib_path()
-  vim.notify('[diffs] downloading libvscode_diff...', vim.log.levels.INFO)
+  vim.notify('[diffs.nvim]: downloading libvscode_diff...', vim.log.levels.INFO)
 
   local cmd = { 'curl', '-fSL', '-o', dest, url }
 
@@ -198,7 +198,7 @@ function M.ensure(callback)
     vim.schedule(function()
       local handle = nil
       if result.code ~= 0 then
-        vim.notify('[diffs] failed to download libvscode_diff', vim.log.levels.WARN)
+        vim.notify('[diffs.nvim]: failed to download libvscode_diff', vim.log.levels.WARN)
         dbg('curl failed: %s', result.stderr or '')
       else
         local f = io.open(version_path(), 'w')
@@ -206,7 +206,7 @@ function M.ensure(callback)
           f:write(EXPECTED_VERSION)
           f:close()
         end
-        vim.notify('[diffs] libvscode_diff downloaded', vim.log.levels.INFO)
+        vim.notify('[diffs.nvim]: libvscode_diff downloaded', vim.log.levels.INFO)
         handle = M.load()
       end