fix: make skill loading instructions more aggressive in system prompt (#8209)

The previous wording ('If one clearly matches') set too high a threshold,
and 'If none match, proceed normally' was an easy escape hatch for lazy
models. Now:

- Lowered threshold: 'matches or is even partially relevant'
- Added MUST directive and 'err on the side of loading' guidance
- Replaced permissive closer with 'only proceed without if genuinely none
  are relevant'

This should reduce cases where the agent skips loading relevant skills
unless explicitly forced.

agent/prompt_builder.py

@@ -726,8 +726,13 @@ def build_skills_system_prompt(
 
         result = (
             "## Skills (mandatory)\n"
-            "Before replying, scan the skills below. If one clearly matches your task, "
-            "load it with skill_view(name) and follow its instructions. "
+            "Before replying, scan the skills below. If a skill matches or is even partially relevant "
+            "to your task, you MUST load it with skill_view(name) and follow its instructions. "
+            "Err on the side of loading — it is always better to have context you don't need "
+            "than to miss critical steps, pitfalls, or established workflows. "
+            "Skills contain specialized knowledge — API endpoints, tool-specific commands, "
+            "and proven workflows that outperform general-purpose approaches. Load the skill "
+            "even if you think you could handle the task with basic tools like web_search or terminal.\n"
             "If a skill has issues, fix it with skill_manage(action='patch').\n"
             "After difficult/iterative tasks, offer to save as a skill. "
             "If a skill you loaded was missing steps, had wrong commands, or needed "
@@ -737,7 +742,7 @@ def build_skills_system_prompt(
             + "\n".join(index_lines) + "\n"
             "</available_skills>\n"
             "\n"
-            "If none match, proceed normally without loading a skill."
+            "Only proceed without loading a skill if genuinely none are relevant to the task."
         )
 
     # ── Store in LRU cache ────────────────────────────────────────────