docs(v2): note session init route removal

specs/v2/session.md

@@ -0,0 +1,17 @@
+# Session API
+
+## Remove Dedicated `session.init` Route
+
+The dedicated `POST /session/:sessionID/init` endpoint exists only as a compatibility wrapper around the normal `/init` command flow.
+
+Current behavior:
+
+- the route calls `SessionPrompt.command(...)`
+- it sends `Command.Default.INIT`
+- it does not provide distinct session-core behavior beyond running the existing init command in an existing session
+
+V2 plan:
+
+- remove the dedicated `session.init` endpoint
+- rely on the normal `/init` command flow instead
+- avoid reintroducing `Session.initialize`-style special cases in the session service layer