chore(server-changes): consolidate sessions PR into one entry

Collapse `session-out-settled-signal.md` and `sessions-public-api-cors.md`
into the single `session-primitive.md`, and rewrite that one to a high-
level two-sentence summary that covers everything actually shipping in
this PR (sessions-as-run-manager, end-and-continue, waitpoints, etc.).
The CORS/JWT-on-create story is also out of date now that POST
/api/v1/sessions is secret-key only.

Author: ericallam

.server-changes/session-out-settled-signal.md

@@ -3,6 +3,4 @@ area: webapp
 type: feature
 ---
 
-Add `Session` primitive — a durable, typed, bidirectional I/O primitive that outlives a single run, intended for agent/chat use cases. Ships the Postgres schema (`Session` table), control-plane CRUD routes (`POST/GET/PATCH /api/v1/sessions`, `POST /api/v1/sessions/:session/close` — polymorphic on friendlyId or externalId), `sessions` JWT scope, ClickHouse `sessions_v1` table, and `SessionsReplicationService` (logical replication from Postgres `Session` → ClickHouse `sessions_v1`). Run-scoped realtime streams (`streams.pipe`/`streams.input`) are unchanged and do **not** create Session rows.
-
-Adds `POST /api/v1/runs/:runFriendlyId/session-streams/wait` (session-stream waitpoint creation) and wires `POST /realtime/v1/sessions/:session/:io/append` to fire any pending waitpoints on the channel. Gives `session.in` run-engine waitpoint semantics matching run-scoped input streams: a task can suspend while idle on a session channel and resume when an external client sends a record. Redis-backed pending-waitpoint set (`ssw:{sessionFriendlyId}:{io}`) is drained atomically on each append so multiple concurrent waiters (e.g. multi-tab chat) all resume together.
+Add the `Session` primitive — a durable, task-bound, bidirectional I/O channel that outlives a single run and acts as the run manager for `chat.agent`. Ships the Postgres `Session` + `SessionRun` tables, ClickHouse `sessions_v1` + replication service, the `sessions` JWT scope, and the public CRUD + realtime routes (`/api/v1/sessions`, `/realtime/v1/sessions/:session/:io`) including `end-and-continue` for server-orchestrated run handoffs and session-stream waitpoints.