Streaming step 7: stop emitting StreamingDeltaEvent and freeze the legacy events endpoint

Part of #4671 (step 7). Needs step 6.

Desired Behavior

Once every client speaks the session endpoint, stop producing the old channel.

Work

1. Stop emitting StreamingDeltaEvent. Deltas become progress frames on the session socket only. This is what structurally retires the two unfiled bugs: nothing left for telemetry to count, nothing left for webhooks to POST. Any interim kind filter added earlier can come out with it.
2. Freeze /sockets/events/{id} and announce its removal. Nothing decodes wrong — it just stops being offered.
3. Remove the legacy resend_mode / after_timestamp replay parameters, superseded by the after_seq cursor.

Why it is last

Every client has to have switched first, which is step 6 plus the two client PRs. Until then the old endpoint is the only thing some clients speak.

Acceptance Criteria

• StreamingDeltaEvent has no construction site.
• A webhook receives the same event count for a streamed and non-streamed conversation, with no filter in the path.
• The legacy endpoint is documented as deprecated with a removal target.