refactor: extract SKILL_PROMPT_DESC_LIMIT constant and normalize description helpers

The system prompt skill index truncates long descriptions to 57 chars,
but this limit was a hardcoded magic number. Extract it as a named
constant and factor the normalization logic into a shared private
helper so the extraction function and the new truncation predicate
cannot drift.

No behaviour change — pure refactor.

agent/skill_utils.py

@@ -779,18 +779,31 @@ def resolve_skill_config_values(
 
 # ── Description extraction ────────────────────────────────────────────────
 
+SKILL_PROMPT_DESC_LIMIT = 60
+
+
+def _normalize_skill_description(frontmatter: Dict[str, Any]) -> str:
+    """Normalize a skill's description field for comparison/truncation."""
+    raw_desc = frontmatter.get("description", "")
+    return str(raw_desc).strip().strip("'\"") if raw_desc else ""
+
 
 def extract_skill_description(frontmatter: Dict[str, Any]) -> str:
-    """Extract a truncated description from parsed frontmatter."""
-    raw_desc = frontmatter.get("description", "")
-    if not raw_desc:
+    """Extract a system-prompt-length description from parsed frontmatter."""
+    desc = _normalize_skill_description(frontmatter)
+    if not desc:
         return ""
-    desc = str(raw_desc).strip().strip("'\"")
-    if len(desc) > 60:
-        return desc[:57] + "..."
+    if len(desc) > SKILL_PROMPT_DESC_LIMIT:
+        return desc[:SKILL_PROMPT_DESC_LIMIT - 3] + "..."
     return desc
 
 
+def is_skill_description_truncated_for_prompt(frontmatter: Dict[str, Any]) -> bool:
+    """True when the description will be truncated in the system prompt skill index."""
+    desc = _normalize_skill_description(frontmatter)
+    return len(desc) > SKILL_PROMPT_DESC_LIMIT
+
+
 # ── File iteration ────────────────────────────────────────────────────────