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.

pyproject.toml

@@ -42,6 +42,7 @@ mail = ["imap-tools>=1.7,<2"]
 # synapse/search.py imports this lazily behind a bare except, so without it
 # declared the chat web-search path silently returns nothing.
 search = ["duckduckgo-search>=6,<9"]
+tui = ["textual>=1.0,<3"]
 desktop = [
   "psutil>=5.9,<8",
   "pywebview>=5,<7; platform_system == 'Windows'",
@@ -64,6 +65,7 @@ all = [
   "faster-whisper>=1.1,<2",
   "imap-tools>=1.7,<2",
   "duckduckgo-search>=6,<9",
+  "textual>=1.0,<3",
   "pywebview>=5,<7; platform_system == 'Windows'",
 ]
 dev = [