agent: make claudePlugins additive instead of replacing

The base set (skill-creator + base@hyperhive) was declared via the
option's `default`, so a per-agent definition of claudePlugins
replaced it wholesale. Move the base set to a plain `config`
definition instead: a listOf option merges multiple plain definitions
by concatenation, so an agent's own list now adds to the base set
rather than replacing it, while lib.mkForce / lib.mkOverride on the
agent side still replace the whole merged list deliberately (mkDefault
was ruled out explicitly).

Also de-dup at the JSON-render site with lib.unique, so an agent that
names a base-set entry itself doesn't get it installed twice, and
reword the option doc, which still claimed the old REPLACES semantics.

Four module-eval cases cover the unset / agent-adds / mkForce-replaces
/ duplicate-entry shapes.

Refs #4467

nix/agent-modules/claude-settings.nix

@@ -190,10 +190,13 @@ in
 
   options.services.hyperhive.agent.claudePlugins = lib.mkOption {
     type = lib.types.listOf lib.types.str;
-    default = [
-      "skill-creator@claude-plugins-official"
-      "base@hyperhive"
-    ];
+    # No `default` here — the base set below is set as a plain `config`
+    # DEFINITION (near the bottom of this file) instead. A
+    # `default` is exactly what a per-agent definition REPLACES; a second
+    # definition at the same option instead MERGES, which for a
+    # `listOf` is a concatenation — the standard NixOS shape for "the base
+    # set plus whatever the agent adds". `lib.mkForce` / `lib.mkOverride`
+    # still let an operator replace the whole merged list deliberately.
     example = [
       "formatter@my-marketplace"
       "thinking-tools@anthropics"
@@ -208,7 +211,7 @@ in
       `/etc/hyperhive/claude-plugins.json`; the harness reads it via
       `plugins::install_configured`.
 
-      Defaults to Anthropic's `skill-creator` (teaches an agent to
+      Always includes Anthropic's `skill-creator` (teaches an agent to
       write, refine, and evaluate its own skills) plus hyperhive's own
       `base` plugin — one plugin bundling every skill that applies to
       *all* agents regardless of role (currently just `state-hygiene`,
@@ -219,11 +222,15 @@ in
       Agents get both out of the box, matching the default
       marketplaces above.
 
-      Note that a per-agent definition REPLACES this default rather
-      than extending it (ordinary NixOS list-option semantics, same as
-      `claudeMarketplaces`). An agent that wants extra plugins AND the
-      defaults should list both `skill-creator@claude-plugins-official`
-      and `base@hyperhive` alongside them.
+      A per-agent definition ADDS to that base set rather than
+      replacing it — unlike `claudeMarketplaces`, this list is merged
+      across every module that defines it (ordinary NixOS list-option
+      semantics: two plain definitions concatenate). An agent that sets
+      `services.hyperhive.agent.claudePlugins = [ "foo@bar" ]` gets the
+      base set plus `foo@bar`, with no need to repeat the base entries.
+      An operator who wants to replace the base set entirely, rather
+      than add to it, can still do that with `lib.mkForce` or
+      `lib.mkOverride`.
     '';
   };
 
@@ -241,8 +248,20 @@ in
   };
 
   config = {
+    # The base set, as a DEFINITION rather than the option's `default` —
+    # see the comment on the option above. A plain definition (no
+    # `lib.mkOrder` wrapper) merges at the same priority as a per-agent
+    # definition, so `lib.mkForce` on the agent side still wins outright.
+    services.hyperhive.agent.claudePlugins = [
+      "skill-creator@claude-plugins-official"
+      "base@hyperhive"
+    ];
+
     environment.etc."hyperhive/claude-plugins.json".text =
-      builtins.toJSON config.services.hyperhive.agent.claudePlugins;
+      # `lib.unique` so an agent that lists `base@hyperhive` (or
+      # `skill-creator@…`) explicitly, on top of the base set this module
+      # already contributes, doesn't install it twice.
+      builtins.toJSON (lib.unique config.services.hyperhive.agent.claudePlugins);
 
     environment.etc."hyperhive/claude-marketplaces.json".text =
       builtins.toJSON config.services.hyperhive.agent.claudeMarketplaces;