Follow-ups on salvaged #85287:
- discover_entrypoint_manifests() now carries BOTH the import-free kind
classification (from #85527) and capability declarations — the two
contracts compose in one function instead of the capability rewrite
dropping classification.
- Per-entry exception isolation: one malformed distribution no longer
blanks every other plugin's manifest (same contract as
providers/__init__.py entry-point scan).
- Documented the hermes_agent.plugin_capabilities group in the plugin
developer guide (pyproject example).
