feat(nvim): add pi.nvim completion source to blink.cmp

Integrate pi-agent as a blink.cmp completion source (score_offset=100)
for LSP-style completions inside pi-chat-prompt buffers. Add the
pi.nvim plugin with a toggle keymap and --approve mode.

config/nvim/plugin/pi.lua

@@ -0,0 +1,9 @@
+vim.pack.add({ "https://github.com/alex35mil/pi.nvim" })
+
+vim.keymap.set({'n'}, '<leader>mp', '<cmd>Pi<cr>',  { noremap = true, silent = true, desc = "Toggle Pi Agent" })
+require("pi").setup({
+    cli = {
+        args = { "--approve" },
+    },
+})
+