feat: add healthcheck

Problem: users had no way to diagnose why completions were missing or
incomplete.

Solution: add a :checkhealth module that verifies blink.cmp is
installed, tmux is on PATH and responds to list-commands, and man is
available for command descriptions.

lua/blink-cmp-tmux/health.lua

@@ -0,0 +1,36 @@
+local M = {}
+
+function M.check()
+  vim.health.start('blink-cmp-tmux')
+
+  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('tmux')
+  if bin ~= '' then
+    vim.health.ok('tmux executable found: ' .. bin)
+  else
+    vim.health.error('tmux executable not found')
+    return
+  end
+
+  local result = vim.system({ 'tmux', 'list-commands' }):wait()
+  if result.code == 0 and result.stdout and result.stdout ~= '' then
+    vim.health.ok('tmux list-commands produces output')
+  else
+    vim.health.warn('tmux list-commands failed (completions will be unavailable)')
+  end
+
+  local man_bin = vim.fn.exepath('man')
+  if man_bin ~= '' then
+    vim.health.ok('man executable found: ' .. man_bin)
+  else
+    vim.health.warn('man executable not found (command descriptions will be unavailable)')
+  end
+end
+
+return M