feat(profiler/behave_shell): emit cognitive.command_branch_diversity

BEHAVE-EXTRACTOR.md Phase A Step 6. Content-based playbook-vs-
adaptive split. Splits CLAUDE-FF (linear_playbook, ~10 distinct
tools) from CLAUDE-CL (adaptive_branching, 5-6 tools with curl
re-invoked) per the 2026-05-02 empirical anchor.

* _features/cognitive.py:command_branch_diversity(ctx) emits one
  Observation in {linear_playbook, adaptive_branching, unknown}.
* unique_first_token_hashes / total_commands ratio. ≥ 0.80 →
  linear_playbook, otherwise adaptive_branching (the doc instructs
  bias-to-adaptive in the middle band — that's the discriminative
  signal we actually want).
* < 5 commands → "unknown" at confidence 1.0 (the absence of data
  is itself a high-confidence answer per the registry's allowed
  vocabulary). Zero-command session skips emission entirely.

Tests cover unique-tokens → linear, repeated-tokens → adaptive,
middle band → adaptive (bias), under-floor → unknown @ 1.0, plus
PII regression: raw tokens never appear in the serialised
observation.

tests/profiler/behave_shell/test_cognitive_command_branch_diversity.py

@@ -0,0 +1,64 @@
+"""Step 6: ``cognitive.command_branch_diversity``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+def _of(observations: list, primitive: str):
+    obs = [o for o in observations if o.primitive == primitive]
+    assert len(obs) == 1, f"expected exactly one {primitive}, got {len(obs)}"
+    return obs[0]
+
+
+def _commands(first_tokens: list[str]) -> list[AsciinemaEvent]:
+    """One command per token, well-spaced."""
+    events: list[AsciinemaEvent] = []
+    t = 0.0
+    for tok in first_tokens:
+        events.append((t, "i", f"{tok} arg\r"))
+        t += 1.0
+    return events
+
+
+def test_under_floor_emits_unknown_high_confidence() -> None:
+    out = list(extract_session(_commands(["ls", "ps", "id"]), sid="bd-low"))
+    obs = _of(out, "cognitive.command_branch_diversity")
+    assert obs.value == "unknown"
+    assert obs.confidence == 1.0
+
+
+def test_unique_first_tokens_emit_linear_playbook() -> None:
+    # 8 distinct tools — ratio 1.0 → linear_playbook
+    tokens = ["uname", "id", "whoami", "pwd", "ls", "ps", "netstat", "ss"]
+    out = list(extract_session(_commands(tokens), sid="bd-linear"))
+    obs = _of(out, "cognitive.command_branch_diversity")
+    assert obs.value == "linear_playbook"
+    assert obs.confidence == 0.80
+
+
+def test_repeated_first_tokens_emit_adaptive_branching() -> None:
+    # 8 commands, only 3 distinct — ratio 0.375 < 0.60
+    tokens = ["curl", "curl", "curl", "ls", "curl", "ls", "curl", "ps"]
+    out = list(extract_session(_commands(tokens), sid="bd-adaptive"))
+    obs = _of(out, "cognitive.command_branch_diversity")
+    assert obs.value == "adaptive_branching"
+
+
+def test_middle_band_biases_to_adaptive() -> None:
+    # 7 commands, 5 unique → ratio ≈ 0.71 — between 0.60 and 0.80.
+    # The doc instructs us to bias to adaptive in the ambiguous middle.
+    tokens = ["a", "b", "c", "d", "e", "a", "b"]
+    out = list(extract_session(_commands(tokens), sid="bd-mid"))
+    obs = _of(out, "cognitive.command_branch_diversity")
+    assert obs.value == "adaptive_branching"
+
+
+def test_pii_no_command_bodies_in_observation() -> None:
+    out = list(extract_session(_commands(
+        ["secret_arg_payload"] * 6,
+    ), sid="bd-pii"))
+    obs = _of(out, "cognitive.command_branch_diversity")
+    # Whatever the verdict, the raw token must not be in the dump
+    serialised = obs.model_dump_json()
+    assert "secret_arg_payload" not in serialised