home-manager/gui: Disable pi-coding-agent (breaks CI)

`pi-coding-agent-bun` breaks `nix flake check` in CI: its bun2nix
`fetchBunDeps` calls `builtins.filterSource` on subpaths of the pi.nix
flake source, which requires that source derivation to be realised in the
local store. A fresh CI runner doesn't have it, so eval aborts with
`path '...-source.drv' is not valid`. It only works locally because the
source is already realised there.

Not using pi right now, so comment out the input, overlays, home module
import and the package (leaving a root-cause note in `flake.nix`) rather
than working around the upstream issue.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nixos/modules/common.nix

@@ -83,7 +83,7 @@ in
           inputs.boardie.overlays.default
           inputs.copyparty.overlays.default
           inputs.hass-west-wood.overlays.default
-          inputs.pi-agent.overlays.default
+          # inputs.pi-agent.overlays.default
         ];
         config = {
           allowUnfree = true;