emacs: add pi-coding-agent package

Add dnouri/pi-coding-agent as an Emacs frontend for the pi coding agent.
Pinned to commit 8d8158b.

- Package installed from GitHub with all .el files (includes md-ts-mode)
- Suppress grammar auto-install prompt since tree-sitter grammars are
  already managed by Nix (treesit-grammars.with-all-grammars)
- Keybinding: SPC o p to launch pi-coding-agent
- Alias: M-x pi as shortcut for M-x pi-coding-agent

home/roles/emacs/doom/packages.el

@@ -72,3 +72,8 @@
   :recipe (:type git :repo "https://codeberg.org/ctietze/beads.el.git"
            :files ("lisp/*.el"))
   :pin "f40a6461d3c0fa0969311bbb6a1e30d1bba86c88")
+
+(package! pi-coding-agent
+  :recipe (:host github :repo "dnouri/pi-coding-agent"
+           :files ("*.el"))
+  :pin "8d8158b0a6150ce13d91e561a1223790670acaa7")