feat: add ThePrimeagen/99 AI agent plugin

- Configured with OpenCodeProvider (default, matches opencode environment)
- blink.cmp as completion source for # rules and @ file references
- tmp_dir = ./tmp (per-project, keeps opencode permission happy)
- Auto-injects AGENT.md files from file location up to project root
- Keymaps under <leader>9 group:
    9v (visual) replace selection with AI result
    9s search project → quickfix list
    9x stop all in-flight requests
    9c clear previous results
    9l view debug logs
    9m telescope model picker
    9p telescope provider picker
- Added AI [9]9 group to which-key

lua/plugins/which-key.lua

@@ -24,6 +24,7 @@ return {
       { '<leader>t', group = '[T]oggle' },
       { '<leader>h', group = 'Git [H]unk', mode = { 'n', 'v' } },
       { '<leader>d', group = '[D]ebug' },
+      { '<leader>9', group = 'AI [9]9' },
     },
   },
 }