feat(profiler/behave_shell): emit environmental.terminal_multiplexer

Scans RAW output (multiplexer escapes are themselves ANSI; never
strip first) for tmux markers (DCS passthrough, focus-reporting,
window-title with tmux marker) and screen markers (DCS, screen-OSC).
Detected → tmux/screen at 0.85; otherwise → none at 0.55. Skips
emission entirely when no commands — silence on a pure-echo or
empty session, per the smoke gates.

When both detected (nested mux), prefer tmux.

decnet/profiler/behave_shell/_features/__init__.py

@@ -26,6 +26,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
 )
 from decnet.profiler.behave_shell._features.environmental import (
     shell_type,
+    terminal_multiplexer,
 )
 from decnet.profiler.behave_shell._features.temporal import (
     escalation_pattern,
@@ -71,4 +72,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     escalation_pattern,
     landing_ritual,
     shell_type,
+    terminal_multiplexer,
 )