[codex] Show skill detail API errors

[franksong2702]

Thinking Path

A small quality pass found the Skills detail pane could silently render (no content) when the API returned HTTP 200 with an error payload such as success: false. The backend fix is covered separately by #2250; this PR adds a UI guard so this class of failure is visible instead of looking like empty skill content.

What Changed

• Added an error renderer for Skills detail panes.
• Treat /api/skills/content responses with success: false or error as failures before rendering markdown content.
• Treat linked skill-file content responses with error as failures as well.
• Added a regression test that verifies the error guard runs before the empty-content fallback.
• Updated the changelog.

Why It Matters

This does not replace the backend root-cause fix. It prevents future regressions where a successful HTTP response contains an application-level error and the UI hides that error behind an empty detail pane.

Before / After

Before: API error payload is hidden and the skill looks empty.

After: the same API error is visible in the Skills detail pane.

Verification

• node --check static/panels.js
• /Users/xuefusong/hermes-webui/.venv/bin/pytest tests/test_skill_detail_error_guard.py -q
• Browser before/after validation with isolated HERMES_HOME and HERMES_WEBUI_STATE_DIR:
  • before on origin/master: selected skill rendered (no content) and no visible backend error
  • after on this branch: selected skill rendered Non-relative patterns are unsupported in the detail pane

Risks / Follow-ups

• This only improves UI behavior for error payloads; it does not change the backend Skills content path. The backend root cause remains covered by [codex] Fix blank skill detail views #2250.
• The error message is intentionally direct because it is already what the API returns.

Model Used

GPT-5 Codex. AI assistance was used to inspect, modify, test, and prepare this pull request.