feat(cli): add interactive TUI chat

Add a Textual chat interface with threaded SSE streaming, slash commands, interrupt handling, and bare nexus dispatch. Package it behind the tui extra, document usage, and cover command routing, dependencies, and headless interaction with tests.

tests/test_packaging_deps.py

@@ -29,7 +29,8 @@ DISTRIBUTION_OF = {
 }
 
 # Provided by another declared distribution rather than named directly.
-TRANSITIVE = {"starlette", "socketio", "engineio"}
+# rich: Textual depends on it, so the tui extra already pulls it in.
+TRANSITIVE = {"starlette", "socketio", "engineio", "rich"}
 
 # Modules that ship inside this repo.
 FIRST_PARTY = {"synapse", "nexusos_cli", "modules", "management", "bin", "tests"}