chore(pi): remove build-source and eval prompt files

These aren't useful for direct invocation — the agent should just follow AGENTS.md instead.

Assisted-by: pi (qwen3.6-35b-a3b)

.pi/prompts/build-source.md

@@ -1,5 +0,0 @@
----
-description: Build source output
-argument-hint: "<attr>"
----
-Build source output: `nix build .#$1.source --no-link --print-out-paths`. Follow AGENTS.md.

.pi/prompts/eval.md

@@ -1,5 +0,0 @@
----
-description: Evaluate flake attr
-argument-hint: "<attr> [extra args]"
----
-Evaluate `.#$1`: `nix eval .#$1 --json ${@:2}`. Follow AGENTS.md.