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.

(cherry picked from commit 66af86e7ee)

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 [];