feat(terminal): tint terminal background per agent color

Add backend-agnostic terminal color support via OSC escape sequences:
- New backend/terminal.py with palette_printf() and exec_shell_script()
  shared by both Docker and smolmachines bottle backends
- Emits OSC 4 (indexed palette) + OSC 11 (default background tint)
  before launching; resets both on agent exit via OSC 104/111
- OSC 11 background tint is visible even when the TUI uses true/24-bit
  colors (which bypass the palette), as Codex does for its chrome
- Fix Codex [tui] config: status_line=["model-with-reasoning"],
  theme="ansi" (dark-ansi and cwd/directory were invalid identifiers)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_agent_provider.py

@@ -80,7 +80,7 @@ class TestAgentProviderRuntime(unittest.TestCase):
         self.assertTrue(plan.has_prompt)
         self.assertEqual("Existing instructions.\n", prompt)
         self.assertIn("[tui]", config)
-        self.assertIn('status_line = ["model", "cwd"]', config)
+        self.assertIn('status_line = ["model-with-reasoning"]', config)
         self.assertIn('terminal_title = ["spinner", "project"]', config)
 
     def test_codex_forward_host_credentials_adds_auth_and_verify(self):