feat: add healthcheck (#9)

* feat: add healthcheck

Problem: users had no way to diagnose why completions were missing or
incomplete beyond checking for the ghostty executable.

Solution: add a :checkhealth module that verifies blink.cmp is
installed, ghostty is on PATH, +show-config --docs produces output,
and the bash completion file exists for enum values.

* fix: revert blanket diagnostics.disable and selene comments

Problem: .luarc.json blanket-disabled four diagnostic categories
project-wide, and selene inline directives were added to suppress
warnings on io.open monkey-patching in tests.

Solution: revert .luarc.json to match main and remove selene comments.

* refactor: reuse main module's bash completion path resolution in healthcheck

Problem: health.lua duplicated the entire bash completion file
resolution chain (exepath -> realpath -> prefix match -> path
construction) from the main module, risking drift if the logic changes.

Solution: extract M.bash_completion_path() from parse_enums() and call
it from both parse_enums and the healthcheck.

lua/blink-cmp-ghostty.lua

@@ -67,21 +67,29 @@ local function parse_keys(stdout)
   return items
 end
 
----@return table<string, string[]>
-local function parse_enums()
+---@return string?
+function M.bash_completion_path()
   local bin = vim.fn.exepath('ghostty')
   if bin == '' then
-    return {}
+    return nil
   end
   local real = vim.uv.fs_realpath(bin)
   if not real then
-    return {}
+    return nil
   end
   local prefix = real:match('(.*)/bin/ghostty$')
   if not prefix then
+    return nil
+  end
+  return prefix .. '/share/bash-completion/completions/ghostty.bash'
+end
+
+---@return table<string, string[]>
+local function parse_enums()
+  local path = M.bash_completion_path()
+  if not path then
     return {}
   end
-  local path = prefix .. '/share/bash-completion/completions/ghostty.bash'
   local fd = io.open(path, 'r')
   if not fd then
     return {}