fix: assert unknown profiles in mkHomeConfig; move agent modules to coding/agents

- home/lib/default.nix: add assertion for unknown profile names instead of
  silently filtering them out; remove unused 'inherit (lib) optional'
- home/coding/agents/{opencode,pi}.nix: moved from home/features/coding/
  to co-locate with agents.nix (eliminating cross-directory back-references)
- home/coding/agents/agents.nix: update imports to ./opencode.nix and ./pi.nix
- home/features/coding/: remove now-dead default.nix (nothing imported it)

home/coding/agents/agents.nix

@@ -19,8 +19,8 @@ in {
 
   imports = [
     # OpenCode and Pi agent configurations
-    ../../features/coding/opencode.nix
-    ../../features/coding/pi.nix
+    ./opencode.nix
+    ./pi.nix
   ];
 
   coding.agents.skills = {