List canonical cross-venue event identities

Keyless cursor-paged directory of active canonical events — CoinRithm's stable cross-venue identity for one real-world question, independent of any single venue's slug.

The identifier is specified as an adoptable standard, including its permanence guarantees and the rule that a merged key never 404s: https://github.com/CoinRithm/coinrithm-agent-trading/blob/main/EVENT_ID_STANDARD.md

Lists ACTIVE canonicals only; merged ones remain resolvable by key via the detail endpoint. A canonical exists only where at least two venues listed the same question, so this is a cross-venue cluster directory rather than a catalogue of every event.