refactor: remove legacy mkOpencodeRules alias and opencode-rules compat entry

- Remove mkOpencodeRules backward-compat alias from lib/coding-rules.nix
- Remove opencode-rules alias from lib/default.nix
- Update shells/opencode.nix to use mkCodingRules / coding-rules
- Remove backward-compat test from tests/lib/coding-rules-test.nix
- Update AGENTS.md and modules/home-manager/AGENTS.md docs
- Apply nix fmt formatting to shared-options.nix

modules/home-manager/coding/agents/shared-options.nix

@@ -4,11 +4,12 @@
   inherit (lib) mkOption mkEnableOption types literalExpression;
 in {
   # Common agentsInput option used by all agent modules.
-  mkAgentsInputOption = description: mkOption {
-    type = types.nullOr types.anything;
-    default = null;
-    inherit description;
-  };
+  mkAgentsInputOption = description:
+    mkOption {
+      type = types.nullOr types.anything;
+      default = null;
+      inherit description;
+    };
 
   # Common modelOverrides option.
   mkModelOverridesOption = mkOption {
@@ -62,7 +63,7 @@ in {
         { src = inputs.skills-anthropic; selectSkills = [ "claude-api" ]; }
         { src = inputs.basecamp; }
       ]
-  '';
+    '';
   };
 
   # Helper to map externalSkills from module config to mkOpencodeSkills format.
@@ -71,5 +72,6 @@ in {
       entry:
         {inherit (entry) src skillsDir;}
         // lib.optionalAttrs (entry.selectSkills != null) {inherit (entry) selectSkills;}
-    ) cfgEntries;
+    )
+    cfgEntries;
 }