fix(types): allow false sentinel in file_cache type annotation

Problem: lua-language-server reported `cast-local-type` because
`file_cache` was annotated as `table<string, string[]>` but stores
`false` as a sentinel for files that failed to open.

Solution: widen the annotation to `table<string, string[]|false>`.

lua/diffs/init.lua

@@ -315,7 +315,7 @@ end
 ---@param hunks diffs.Hunk[]
 ---@param max_lines integer
 local function compute_hunk_context(hunks, max_lines)
-  ---@type table<string, string[]>
+  ---@type table<string, string[]|false>
   local file_cache = {}
 
   for _, hunk in ipairs(hunks) do