fix(cron): warn and skip missing skills instead of crashing job

When a cron job references a skill that is no longer installed,
_build_job_prompt() now logs a warning and injects a user-visible notice
into the prompt instead of raising RuntimeError. The job continues with
any remaining valid skills and the user prompt.

Adds 4 regression tests for missing skill handling.

cron/scheduler.py

@@ -207,11 +207,14 @@ def _build_job_prompt(job: dict) -> str:
     from tools.skills_tool import skill_view
 
     parts = []
+    skipped: list[str] = []
     for skill_name in skill_names:
         loaded = json.loads(skill_view(skill_name))
         if not loaded.get("success"):
             error = loaded.get("error") or f"Failed to load skill '{skill_name}'"
-            raise RuntimeError(error)
+            logger.warning("Cron job '%s': skill not found, skipping — %s", job.get("name", job.get("id")), error)
+            skipped.append(skill_name)
+            continue
 
         content = str(loaded.get("content") or "").strip()
         if parts:
@@ -224,6 +227,15 @@ def _build_job_prompt(job: dict) -> str:
             ]
         )
 
+    if skipped:
+        notice = (
+            f"[SYSTEM: The following skill(s) were listed for this job but could not be found "
+            f"and were skipped: {', '.join(skipped)}. "
+            f"Start your response with a brief notice so the user is aware, e.g.: "
+            f"'⚠️ Skill(s) not found and skipped: {', '.join(skipped)}']"
+        )
+        parts.insert(0, notice)
+
     if prompt:
         parts.extend(["", f"The user has provided the following instruction alongside the skill invocation: {prompt}"])
     return "\n".join(parts)