Content events omit end entirely when open-ended, but the docs promise an empty value

[galjos]

docs/json-contracts.md:89 and skills/open-data-hub-cli/SKILL.md:99 both say "an empty end means ongoing". The key is not empty, it is absent:

$ odh traffic today --source content --json
of 101 events: key absent 32, key present but null 0

end carries omitempty, so an open-ended announcement has no end at all. A consumer following the documented contract with event["end"] gets a KeyError on exactly the records the sentence is about — the open ones. An agent hit this during a field test of v0.5.0.

Same shape as the outlier key issue fixed in pitstop v1.1.0: a conditionally-present key documented as conditionally-empty.

Two ways out

1. Emit "end": null for content events, matching the docs. Changes the wire shape for a field that is currently absent, so it is additive for anyone using .get() and fixes anyone using [].
2. Keep omitempty and correct both docs to say the key is absent when open-ended.

(1) is better here: end is a documented stable field, and absent-means-ongoing is a subtler contract than null-means-ongoing. But (2) is honest and cheaper.

Done when

• The docs and the emitted JSON agree.
• A test asserts whichever shape is chosen, so they cannot drift again.