Fixes #11200

Problem

build_skill_invocation_message() returned a truthy placeholder string ([Failed to load skill: <name>]) when a registered skill's SKILL.md could not be loaded (e.g. file deleted or corrupted between scan and invocation). Every caller used if msg: to distinguish success from failure, so the placeholder was treated as a successful invocation and silently fed into the agent as user prompt content. A missing/deleted/corrupt skill therefore looked like a working slash command from the user's perspective, while the model received [Failed to load skill: ...] as conversation input.

Affected call sites flagged in the bug report:

• agent/skill_commands.py:311-313
• cli.py:5571-5580
• gateway/run.py:2877-2882
• gateway/platforms/webhook.py:383-388

Fix

• build_skill_invocation_message() now returns None on load failure (matching the existing "unknown skill" path) and logs a warning with the cmd_key + skill_dir for diagnosis. The docstring is updated to document the new contract.
• gateway/run.py: when the slash command resolves to a registered skill but loading fails, return a real error message to the user (instead of silently falling through and sending the bare /cmd to the LLM as free text).
• gateway/platforms/webhook.py: when a configured route skill is registered but fails to load, log loudly at ERROR and skip that skill rather than injecting the placeholder string into the prompt.
• cli.py already had if not msg: branches for both the skill slash handler and _handle_plan_command; those branches were effectively dead code under the old return contract and now fire correctly on real load failures.

Reproduction

The reporter's reproduction now returns None instead of '[Failed to load skill: demo]':

import agent.skill_commands as sc
from pathlib import Path
from tempfile import TemporaryDirectory
from unittest.mock import patch

with TemporaryDirectory() as td:
    root = Path(td)
    skill_dir = root / "demo"
    skill_dir.mkdir(parents=True)
    (skill_dir / "SKILL.md").write_text("---
name: demo
description: demo
---

# Demo
", encoding="utf-8")
    with patch("agent.skill_utils.get_external_skills_dirs", return_value=[root]), \
         patch("tools.skills_tool.SKILLS_DIR", Path(td) / "missing_local_skills"):
        sc.scan_skill_commands()
        (skill_dir / "SKILL.md").unlink()
        print(sc.build_skill_invocation_message("/demo", "hello"))  # -> None

Tests

• New TestBuildSkillInvocationMessage::test_returns_none_when_payload_load_fails_after_scan covers the regression: scan a skill, delete its SKILL.md, assert build_skill_invocation_message returns None.
• pytest tests/agent/test_skill_commands.py — 27 passed.
• pytest tests/gateway/test_webhook_integration.py — 4 passed.