orchestra_audit_trail: config schema is missing the events.task.cancelled key

Config Inspector reports a "missing schema" error for orchestra_audit_trail: events.task.cancelled is "not a supported key".

The task channel emits a "cancelled" audit event (the inbox cancels a work item when its token is cancelled), and the settings form lists a "Cancelled" checkbox for the task channel, so saving the form writes events.task.cancelled to the active config. But the config schema, and the install defaults, only declared the task actions up to "reoffered", so the saved key fails schema validation.

Fix: add cancelled to the task events in config/schema/orchestra_audit_trail.schema.yml and to config/install/orchestra_audit_trail.settings.yml (default on, matching the instance and token cancelled events).

Why CI did not catch it: the strict config-schema check validates only config installed or saved during a test. The install default never carried the key (it stopped at reoffered), and no test saves the settings form, which is the only writer of events.task.cancelled, so the invalid key was never produced in a test. A regression test that saves the full settings tree closes that gap.