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.

lua/blink-cmp-ghostty/health.lua

@@ -0,0 +1,48 @@
+local M = {}
+
+function M.check()
+  vim.health.start('blink-cmp-ghostty')
+
+  local ok = pcall(require, 'blink.cmp')
+  if ok then
+    vim.health.ok('blink.cmp is installed')
+  else
+    vim.health.error('blink.cmp is not installed')
+  end
+
+  local bin = vim.fn.exepath('ghostty')
+  if bin ~= '' then
+    vim.health.ok('ghostty executable found: ' .. bin)
+  else
+    vim.health.error('ghostty executable not found')
+    return
+  end
+
+  local result = vim.system({ 'ghostty', '+show-config', '--docs' }):wait()
+  if result.code == 0 and result.stdout and result.stdout ~= '' then
+    vim.health.ok('ghostty +show-config --docs produces output')
+  else
+    vim.health.warn('ghostty +show-config --docs failed (config key documentation will be unavailable)')
+  end
+
+  local real = vim.uv.fs_realpath(bin)
+  if not real then
+    vim.health.warn('could not resolve ghostty symlink (enum completions will be unavailable)')
+    return
+  end
+  local prefix = real:match('(.*)/bin/ghostty$')
+  if not prefix then
+    vim.health.warn('ghostty binary is not in a standard bin/ directory (enum completions will be unavailable)')
+    return
+  end
+  local path = prefix .. '/share/bash-completion/completions/ghostty.bash'
+  local fd = io.open(path, 'r')
+  if fd then
+    fd:close()
+    vim.health.ok('bash completion file found: ' .. path)
+  else
+    vim.health.warn('bash completion file not found at ' .. path .. ' (enum completions will be unavailable)')
+  end
+end
+
+return M