fix: include available_skills in isolated cron agentTurn sessions (closes #24888)

buildSkillsSection() had an early-return guard on isMinimal that silently
dropped the entire <available_skills> block for any session using
promptMode="minimal" — which includes all isolated cron agentTurn sessions
(isCronSessionKey → promptMode="minimal" in attempt.ts:497-500).

Fix: remove the isMinimal guard from buildSkillsSection so that skills are
emitted whenever a non-empty skillsPrompt is provided, regardless of mode.
Memory, docs, reply-tags, and other verbose sections remain gated on isMinimal.

Tests added:
- "includes skills in minimal prompt mode when skillsPrompt is provided (cron regression)"
- "omits skills in minimal prompt mode when skillsPrompt is absent"
- Updated existing minimal-mode test expectation to match corrected behaviour.

src/agents/system-prompt.test.ts

@@ -108,7 +108,8 @@ describe("buildAgentSystemPrompt", () => {
     });
 
     expect(prompt).not.toContain("## Authorized Senders");
-    expect(prompt).not.toContain("## Skills");
+    // Skills are included even in minimal mode when skillsPrompt is provided (cron sessions need them)
+    expect(prompt).toContain("## Skills");
     expect(prompt).not.toContain("## Memory Recall");
     expect(prompt).not.toContain("## Documentation");
     expect(prompt).not.toContain("## Reply Tags");
@@ -131,6 +132,29 @@ describe("buildAgentSystemPrompt", () => {
     expect(prompt).toContain("Subagent details");
   });
 
+  it("includes skills in minimal prompt mode when skillsPrompt is provided (cron regression)", () => {
+    // Isolated cron sessions use promptMode="minimal" but must still receive skills.
+    const skillsPrompt =
+      "<available_skills>\n  <skill>\n    <name>demo</name>\n  </skill>\n</available_skills>";
+    const prompt = buildAgentSystemPrompt({
+      workspaceDir: "/tmp/openclaw",
+      promptMode: "minimal",
+      skillsPrompt,
+    });
+
+    expect(prompt).toContain("## Skills (mandatory)");
+    expect(prompt).toContain("<available_skills>");
+  });
+
+  it("omits skills in minimal prompt mode when skillsPrompt is absent", () => {
+    const prompt = buildAgentSystemPrompt({
+      workspaceDir: "/tmp/openclaw",
+      promptMode: "minimal",
+    });
+
+    expect(prompt).not.toContain("## Skills");
+  });
+
   it("includes safety guardrails in full prompts", () => {
     const prompt = buildAgentSystemPrompt({
       workspaceDir: "/tmp/openclaw",

src/agents/system-prompt.ts

@@ -22,9 +22,6 @@ function buildSkillsSection(params: {
   isMinimal: boolean;
   readToolName: string;
 }) {
-  if (params.isMinimal) {
-    return [];
-  }
   const trimmed = params.skillsPrompt?.trim();
   if (!trimmed) {
     return [];