feat: add healthcheck (#7)

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.

.luarc.json

@@ -2,6 +2,12 @@
   "runtime.version": "Lua 5.1",
   "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
   "diagnostics.globals": ["vim", "jit"],
+  "diagnostics.disable": [
+    "undefined-doc-name",
+    "undefined-doc-class",
+    "undefined-field",
+    "need-check-nil"
+  ],
   "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
   "workspace.checkThirdParty": false,
   "completion.callSnippet": "Replace"