Document which endpoints are not probed

[bdpedigo]

Every probe is read-only. Many CAVE outages happen on the write and ingest path (edits, materialization runs, annotation posts). A green board can coexist with a broken write path, and operators may read green as "everything is fine." The scope should be stated rather than implied.

Approach

• Keep an explicit list of endpoints and capabilities that echo does not probe, the write path in particular. Surface it in the UI (for example a greyed "write path: not monitored" section) and in the README, so green is not read as "CAVE is healthy."
• This list could be registered but disabled probes, possibly with some description of why they are disabled.
• --dry-run versions of ingest endpoints were discussed and are out of scope here.

Acceptance criteria

• A maintained "not probed" list exists in the repo.
• The UI states the read-availability scope.