Add smoke tests: Lemmy / Kbin

[sij411]

Background

Part of the phase 3 interoperability smoke test rollout tracked in #682 .

Lemmy is a link-aggregator-style ActivityPub server where the primary
federation unit is the community, represented as a Group actor.
Posts are federated as Announce(Create(Note)) — the community wraps
and re-broadcasts content to its followers. Kbin is no longer actively
maintained; Mbin is the preferred
active fork and may be used as an alternative target.

Docker image requirements

• Deployable via Docker Compose with no manual steps beyond configuration
• Exposes an HTTP endpoint reachable by the Caddy TLS proxy within the
  smoke Docker network
• Supports ActivityPub federation out of the box
• Allows programmatic account creation and API token issuance without
  requiring email verification
• Note: Lemmy does not implement the Mastodon-compatible API.
  The provisioning script and any new scenarios must use the Lemmy API
  (/api/v3/*) directly

Test scenarios

The six baseline scenarios in orchestrator.ts do not apply here since
Lemmy does not implement the Mastodon-compatible API. The following new
scenarios are required:

• Fedify → Lemmy (Follow a community): follow a Lemmy community
  (Group actor) from the Fedify harness and verify Accept(Follow) is
  received
• Lemmy → Fedify (Announce(Create(Note))): post to a Lemmy
  community and verify the harness inbox receives an
  Announce(Create(Note)) activity

The harness inbox listener currently records only top-level activity
types. It needs to be extended to unwrap Announce and inspect the
inner object for the second scenario.

Acceptance criteria

• test/smoke/lemmy/ is added with the required files:
  docker-compose.yml, Caddyfile.lemmy, Caddyfile.fedify-harness,
  generate-certs.sh, provision.sh, and a server config file
• The provisioning script creates a test user, obtains an API token, and
  resolves the Fedify harness account
• Both scenarios above pass reliably
• A GitHub Actions workflow (.github/workflows/smoke-lemmy.yml) is
  added, triggered on pushes to main, next, and *.*-maintenance
  branches, and on workflow_dispatch