`integrations/ocm/`: receive Open Cloud Mesh shares (phase 2)

[michielbdejong]

Repo: ontola/atomic-plugins. Base branch: main.
Design: docs/design/server-plugin-routes.md (atomic-plugins PR #108, for atomic-plugins#88), sections 3, 4 (phase 2).
Depends on: ontola/atomic-server#1716 (the ocm discovery claim), ontola/atomic-server#1717, ontola/atomic-server#1718, #134.
Needs --plugin-routes read-write.

Scope

Discovery, shares, notifications and invite-accepted; received shares
listed in a drive table. Evidence: a share from a Nextcloud peer.

From the design in #108 (docs/design/server-plugin-routes.md, #88). Filed by the atomic-plugins oversight session.

[michielbdejong]

Per Michiel's decision, the server-protocol plugins are owned by the Codex PRs; this is covered by #159 (OCM). The atomic-plugins oversight session won't implement it separately.