fix(imessage): match WhatsApp's per-group systemPrompt resolution semantic

Earlier draft used a simple normalize-and-fall-through resolver: any
empty/whitespace per-chat_id systemPrompt fell through to the
groups['*'] wildcard. That doesn't match the WhatsApp resolver
(extensions/whatsapp/src/system-prompt.ts), where defining the
systemPrompt key on the specific group (even as '') means 'this group
has no prompt' and suppresses the wildcard.

Make iMessage resolution byte-identical:
- specific != null && specific.systemPrompt != null -> use the trimmed
  specific (empty trim -> undefined, wildcard suppressed).
- otherwise -> trimmed wildcard.

Add the resolver as a small exported helper resolveIMessageGroupSystemPrompt
so the unit tests cover it directly. Update the per-group systemPrompt
doc section in docs/channels/imessage.md to copy WhatsApp's resolution
hierarchy language and add an explicit-suppression example.

Refs #78285.

docs/channels/imessage.md

@@ -273,7 +273,10 @@ If SIP-disabled isn't acceptable for your threat model:
 
     Per-group `systemPrompt`:
 
-    Each entry under `channels.imessage.groups.*` accepts an optional `systemPrompt` string. The value is injected into the agent's system prompt on every turn that handles a message in that group. The wildcard `groups["*"]` entry is honored as a fallback when the per-`chat_id` entry has no `systemPrompt`.
+    Each entry under `channels.imessage.groups.*` accepts an optional `systemPrompt` string. The value is injected into the agent's system prompt on every turn that handles a message in that group. Resolution mirrors the per-group prompt resolution used by `channels.whatsapp.groups`:
+
+    1. **Group-specific system prompt** (`groups["<chat_id>"].systemPrompt`): used when the specific group entry exists in the map **and** its `systemPrompt` key is defined. If `systemPrompt` is an empty string (`""`) the wildcard is suppressed and no system prompt is applied to that group.
+    2. **Group wildcard system prompt** (`groups["*"].systemPrompt`): used when the specific group entry is absent from the map entirely, or when it exists but defines no `systemPrompt` key.
 
     ```json5
     {
@@ -287,6 +290,10 @@ If SIP-disabled isn't acceptable for your threat model:
               requireMention: true,
               systemPrompt: "This is the on-call rotation chat. Keep replies under 3 sentences.",
             },
+            "9907": {
+              // explicit suppression: the wildcard "Use British spelling." does not apply here
+              systemPrompt: "",
+            },
           },
         },
       },

extensions/imessage/src/monitor/inbound-processing.systemPrompt.test.ts

@@ -92,10 +92,14 @@ describe("resolveIMessageInboundDecision per-group systemPrompt", () => {
     expect(decision.groupSystemPrompt).toBe("Specific group voice.");
   });
 
-  it("treats whitespace-only systemPrompt as undefined", () => {
+  it("treats whitespace-only per-chat_id systemPrompt as suppression of the wildcard", () => {
+    // Mirrors WhatsApp semantic: defining the systemPrompt key on a specific
+    // group entry (even as whitespace) means "this group has no prompt" and
+    // suppresses the groups["*"] fallback.
     const decision = resolveIMessageInboundDecision(
       buildDecisionParams({
         cfg: buildCfgWithGroups({
+          "*": { systemPrompt: "Wildcard." },
           "7": { systemPrompt: "   " },
         }),
       }),
@@ -105,6 +109,34 @@ describe("resolveIMessageInboundDecision per-group systemPrompt", () => {
     expect(decision.groupSystemPrompt).toBeUndefined();
   });
 
+  it("treats explicit empty-string per-chat_id systemPrompt as suppression of the wildcard", () => {
+    const decision = resolveIMessageInboundDecision(
+      buildDecisionParams({
+        cfg: buildCfgWithGroups({
+          "*": { systemPrompt: "Wildcard." },
+          "7": { systemPrompt: "" },
+        }),
+      }),
+    );
+    expect(decision.kind).toBe("dispatch");
+    if (decision.kind !== "dispatch") return;
+    expect(decision.groupSystemPrompt).toBeUndefined();
+  });
+
+  it("falls back to the wildcard when the per-chat_id entry has no systemPrompt key at all", () => {
+    const decision = resolveIMessageInboundDecision(
+      buildDecisionParams({
+        cfg: buildCfgWithGroups({
+          "*": { systemPrompt: "Wildcard." },
+          "7": { requireMention: true },
+        }),
+      }),
+    );
+    expect(decision.kind).toBe("dispatch");
+    if (decision.kind !== "dispatch") return;
+    expect(decision.groupSystemPrompt).toBe("Wildcard.");
+  });
+
   it("does not set groupSystemPrompt on true DM decisions", () => {
     // Use a chat_id that does NOT match any configured group entry, and
     // route through the DM-shaped message (is_group=false, no chat_id key

extensions/imessage/src/monitor/inbound-processing.ts

@@ -28,7 +28,6 @@ import {
   resolveDmGroupAccessWithLists,
   evaluateSupplementalContextVisibility,
 } from "openclaw/plugin-sdk/security-runtime";
-import { normalizeOptionalString } from "openclaw/plugin-sdk/text-runtime";
 import { sanitizeTerminalText } from "openclaw/plugin-sdk/text-runtime";
 import { truncateUtf16Safe } from "openclaw/plugin-sdk/text-runtime";
 import { resolveIMessageConversationRoute } from "../conversation-route.js";
@@ -132,6 +131,31 @@ function hasIMessageEchoMatch(params: {
   return false;
 }
 
+/**
+ * Per-group `systemPrompt` resolution. Mirrors `resolveWhatsAppGroupSystemPrompt`
+ * in `extensions/whatsapp/src/system-prompt.ts`:
+ *
+ * 1. If the matched per-`chat_id` entry exists AND defines `systemPrompt` (key
+ *    is present, value is non-null), use it. Trim whitespace; if the trim
+ *    leaves an empty string, return `undefined` and DO NOT fall through to the
+ *    wildcard. This is how operators say "this specific group has no prompt"
+ *    without inheriting from `groups["*"]`.
+ * 2. Otherwise, return the wildcard `groups["*"].systemPrompt` (trimmed; empty
+ *    after trim → `undefined`).
+ */
+export function resolveIMessageGroupSystemPrompt(params: {
+  groupConfig: unknown;
+  defaultConfig: unknown;
+}): string | undefined {
+  const specific = params.groupConfig as { systemPrompt?: string | null } | undefined;
+  if (specific != null && specific.systemPrompt != null) {
+    return specific.systemPrompt.trim() || undefined;
+  }
+  const wildcard = (params.defaultConfig as { systemPrompt?: string | null } | undefined)
+    ?.systemPrompt;
+  return wildcard != null ? wildcard.trim() || undefined : undefined;
+}
+
 type IMessageInboundDispatchDecision = {
   kind: "dispatch";
   isGroup: boolean;
@@ -532,16 +556,15 @@ export function resolveIMessageInboundDecision(params: {
   }
 
   // Per-chat_id `systemPrompt` wins; fall back to the `groups["*"]` wildcard
-  // entry when the matched group has no explicit prompt. Mirrors the per-group
-  // systemPrompt pattern already supported by Telegram, IRC, Discord, Slack,
-  // GoogleChat, and the retired BlueBubbles channel.
+  // ONLY when the matched group does not define the key at all. If the matched
+  // group sets `systemPrompt: ""` the wildcard is suppressed (no prompt is
+  // applied to that specific group). Mirrors the resolution semantic in
+  // `extensions/whatsapp/src/system-prompt.ts`.
   const groupSystemPrompt = isGroup
-    ? (normalizeOptionalString(
-        (groupListPolicy.groupConfig as { systemPrompt?: unknown } | undefined)?.systemPrompt,
-      ) ??
-      normalizeOptionalString(
-        (groupListPolicy.defaultConfig as { systemPrompt?: unknown } | undefined)?.systemPrompt,
-      ))
+    ? resolveIMessageGroupSystemPrompt({
+        groupConfig: groupListPolicy.groupConfig,
+        defaultConfig: groupListPolicy.defaultConfig,
+      })
     : undefined;
 
   return {