refactor(config): nest integration toggles under integrations namespace

Problem: integration keys (`fugitive`, `neogit`, `gitsigns`, `committia`,
`telescope`) live at the top level of `vim.g.diffs`, cluttering the
config alongside unrelated options like `highlights` and `conflict`.

Solution: move them under `vim.g.diffs.integrations.*`. Old top-level
keys are migrated automatically with a `vim.deprecate` warning targeting
v0.3.2. `compute_filetypes` and `plugin/diffs.lua` fall back to legacy
keys for pre-`init()` callers.

spec/neogit_integration_spec.lua

@@ -1,6 +1,6 @@
 require('spec.helpers')
 
-vim.g.diffs = { neogit = true }
+vim.g.diffs = { integrations = { neogit = true } }
 
 local diffs = require('diffs')
 local parser = require('diffs.parser')