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/lib/default.nix

@@ -11,8 +11,6 @@
 { lib }:
 
 let
-  inherit (lib) optional;
-
   # Infrastructure layer — nixpkgs overlays, nix-colors, m3ta-nixpkgs modules.
   # Always loaded on every host.
   commonModule = ../common;
@@ -55,6 +53,12 @@ in {
       else if context == "server" then [ contextModuleMap.server ]
       else [];
 
+    # Partition profiles into known and unknown for assertion + safe import.
+    unknownProfiles = builtins.filter
+      (profileName: ! builtins.hasAttr profileName profileModuleMap)
+      profiles;
+
+    # Only import known profiles; the assertion below catches unknowns.
     activeProfiles = builtins.filter
       (profileName: builtins.hasAttr profileName profileModuleMap)
       profiles;
@@ -76,6 +80,14 @@ in {
           "m3ta home: context must be 'desktop', 'server', or null"
           + " (got: '${contextStr}')";
       }
+      {
+        assertion = unknownProfiles == [];
+        message =
+          "m3ta home: unknown profiles requested:"
+          + " [ ${builtins.concatStringsSep " " unknownProfiles} ]."
+          + " Valid profiles are:"
+          + " [ ${builtins.concatStringsSep " " (builtins.attrNames profileModuleMap)} ]";
+      }
     ];
   };
 }