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.
## Problem
Users had to pass `enabled = true` or `enabled = false` inside
fugitive/neogit config tables, which was redundant — table presence
already implied the integration should be active.
## Solution
Remove the `enabled` field from the public API. Table presence now
implies enabled, `false` disables, `true` expands to sub-defaults.
The `enabled` field is still accepted for backward compatibility.
Added 20 `compute_filetypes` tests covering all config shapes (true,
false, table, nil, backward-compat enabled field). Updated docs
and type annotations.
