The interim index (#84919) merged while this PR was open, leaving two
side-by-side discovery systems. Rip the index out, keep only what the
catalog track uses:
- delete hermes_cli/plugin_index.py, the bundled seed JSON, its test
file, the .gitignore exception, and the plugins.index_url surface
- cmd_search is catalog-only (term + --json); --capability/--refresh die
- cmd_install: unknown bare catalog-like names hard-error again instead
of falling through the removed index fallback
- plugin packs bare-name resolution rewired onto plugin_catalog
(load_catalog_live), the only shipped index consumer
- docs (features/plugins.md, cli-commands.md) rewritten for the catalog
46ab5aa365