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,
 )

decnet/profiler/behave_shell/_features/environmental.py

@@ -6,12 +6,31 @@ prompt-line detector. F.0 itself emits no primitive — it populates
 which F.1 / F.3 / E.4 read.
 
 Step F.1: ``environmental.shell_type``.
+Step F.2: ``environmental.terminal_multiplexer``.
 """
 from __future__ import annotations
 
 import collections
 from typing import Iterator
 
+# Multiplexer fingerprints scanned over RAW output (multiplexer escapes
+# ARE ANSI sequences, so we must NOT strip-ANSI before searching).
+# Sources:
+#   tmux DCS passthrough: ESC P tmux ;
+#   tmux focus reporting: ESC [ ? 1004 (set/reset)
+#   tmux window-title with explicit tmux marker
+#   screen DCS:           ESC P =
+#   screen-specific OSC:  ESC ] 83 ;
+_TMUX_MARKERS: tuple[str, ...] = (
+    "\x1bPtmux;",
+    "\x1b[?1004",
+    "\x1b]2;tmux",
+)
+_SCREEN_MARKERS: tuple[str, ...] = (
+    "\x1bP=",
+    "\x1b]83;",
+)
+
 from decnet_behave_core.spec.envelope import Observation
 
 from decnet.profiler.behave_shell._ctx import SessionContext
@@ -73,3 +92,50 @@ def shell_type(ctx: SessionContext) -> Iterator[Observation]:
         value=value,
         confidence=confidence,
     )
+
+
+def terminal_multiplexer(ctx: SessionContext) -> Iterator[Observation]:
+    """Emit ``environmental.terminal_multiplexer`` ∈ {none, tmux, screen}.
+
+    Scans raw output (NOT ANSI-stripped — multiplexer escapes ARE ANSI
+    sequences) for tmux/screen-specific fingerprints. If both detected,
+    prefer tmux (more common in 2026 nested-mux setups). Even one
+    escape is conclusive — no sample-size floor.
+
+    Confidence 0.85 when a fingerprint matches; 0.55 for ``none`` (a
+    bare PTY genuinely has no multiplexer, but a hidden multiplexer
+    that suppresses its escapes would also yield ``none``).
+
+    Skip emission when the session has no commands — without operator
+    interaction the engine should not emit operator-derived primitives.
+    The smoke gates (``test_extract_session_empty_stream_yields_no_observations``,
+    ``test_extract_session_zero_inputs_yields_nothing``) bind this:
+    no commands, no observations.
+    """
+    if not ctx.commands:
+        return
+    has_tmux = False
+    has_screen = False
+    for _t, _k, data in ctx.output_events:
+        if not has_tmux and any(m in data for m in _TMUX_MARKERS):
+            has_tmux = True
+        if not has_screen and any(m in data for m in _SCREEN_MARKERS):
+            has_screen = True
+        if has_tmux and has_screen:
+            break
+
+    if has_tmux:
+        value = "tmux"
+        confidence = 0.85
+    elif has_screen:
+        value = "screen"
+        confidence = 0.85
+    else:
+        value = "none"
+        confidence = 0.55
+    yield make_observation(
+        ctx,
+        primitive="environmental.terminal_multiplexer",
+        value=value,
+        confidence=confidence,
+    )