Two new CLI subcommands that decouple model additions from npm release cadence:

- `external-agents refresh [--url URL]` — pull latest bundled agents.yaml
  from GitHub main (or --url) and write an override to
  ~/.local/state/external-agents/agents.yaml.override.
- `external-agents add-model --id --provider --url --model --env [--tier
  --tags --auth]` — append/replace a locally-authored entry in
  ~/.local/state/external-agents/agents.local.yaml.

Registry loader now layers three sources: bundled → refresh override →
local additions. Same-id replaces, new-id appends. Bundled remains the
canonical source of truth — refresh is an explicit opt-in.

README: new "Keeping the list fresh" section under Providers.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>