refactor: remove ENABLED_PLUGINS config, use PLUGINS as the single plugin selector

ENABLED_PLUGINS and PLUGINS were two near-identical config keys: PLUGINS
was the CLI/per-run whitelist (--plugins flag, runner config), while
ENABLED_PLUGINS was the UI/API "persisted enabled set" — but both ended
up steering the same plugin resolution. Consolidating on PLUGINS as the
single source of truth.

- archivebox/config/common.py: drop the ENABLED_PLUGINS Field entirely
  (no alias, no compat shim — the migration is one-shot).
- archivebox/hooks.py:get_enabled_plugins(): read PLUGINS instead of
  ENABLED_PLUGINS. Function name kept (describes the return value).
- archivebox/templates/core/add.html: admin "Add" form JS now writes to
  PLUGINS; help text updated to reference PLUGINS.

views.py:1302 and runner.py:585 already read/wrote PLUGINS; they're now
consistent with the resolver.

abx-dl is unaffected — it receives selected_plugins as a Python list
argument and never reads either config key.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>