Treat failed skill loads as actual command failures

[cola-runner]

Closes #11200

Summary

• return None when a cached skill command can no longer load its payload, instead of injecting a bracketed placeholder into the model prompt
• surface known skill load failures to gateway users so broken /skill commands do not fall through to the agent as plain text
• add regressions for post-scan skill deletion plus CLI/gateway failure handling

Testing

• source venv/bin/activate && python -m pytest tests/agent/test_skill_commands.py tests/gateway/test_unknown_command.py tests/cli/test_cli_prefix_matching.py tests/gateway/test_webhook_integration.py -q
• source venv/bin/activate && python -m pytest tests/ -q

Full tests/ -q still reflects the current repo baseline in this environment: 73 failed, 12120 passed, 104 skipped, 50 errors, dominated by existing ACP / FastAPI web / gateway / run_agent failures unrelated to this change.

[cola-runner]

Closing this as a duplicate of the earlier fixes already linked on #11200 (#11380 and #11408). I missed the existing linked PRs before sending this one up, so I'm clearing it out to avoid duplicate review traffic.