fix(catalog): remove the parallel community plugin index — the catalog is the only discovery system

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

.gitignore

@@ -35,7 +35,6 @@ data/
 # Bundled community plugin index seed (shipped as package data) — the bare
 # `data/` pattern above would otherwise swallow it.
 !hermes_cli/data/
-!hermes_cli/data/plugin_index.json
 .pytest_cache/
 test_durations.json
 .pytest-cache/