feat(profiler/behave_shell): output error-signal helper for Phase D

Lifts the error-signal slice of F.0 forward as a D.0 prelude. ANSI
strip + canonical bash/sh error fingerprints classify each command's
post-execution output window; Command gains errored / output_bytes
fields. PII discipline preserved — only a bool and an int leave the
helper, the stripped output text is dropped on return.

Drives D.1 (cognitive_load error_rate term) and D.5–D.7 (error_resilience
family). Phase F.0 will subsume this with PS1 + exit-code parsing.

tests/profiler/behave_shell/test_command_error_detection.py

@@ -0,0 +1,134 @@
+"""Step D.0: per-command error-signal helper.
+
+The helper is infrastructure (no primitive emit) — these tests pin
+``Command.errored`` and ``Command.output_bytes`` semantics directly via
+``build_session_context``. The Phase D primitives (D.1, D.5–D.7) all
+read the same fields, so any drift here breaks four downstream
+primitives at once.
+"""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._ctx import build_session_context
+from decnet.profiler.behave_shell._parse import (
+    AsciinemaEvent,
+    detect_error_in_output,
+    strip_ansi,
+)
+
+
+def _typed(text: str, t0: float = 0.0, dt: float = 0.05) -> list[AsciinemaEvent]:
+    return [(t0 + i * dt, "i", c) for i, c in enumerate(text)]
+
+
+# ── strip_ansi ──────────────────────────────────────────────────────────────
+
+
+def test_strip_ansi_removes_csi_sgr() -> None:
+    assert strip_ansi("\x1b[31mPermission denied\x1b[0m") == "Permission denied"
+
+
+def test_strip_ansi_removes_osc_with_bel() -> None:
+    # OSC: ESC ] ... BEL — terminal title escape
+    assert strip_ansi("\x1b]0;title\x07hello") == "hello"
+
+
+def test_strip_ansi_passthrough_clean_text() -> None:
+    assert strip_ansi("plain output\nwith newline") == "plain output\nwith newline"
+
+
+# ── detect_error_in_output ──────────────────────────────────────────────────
+
+
+def test_detect_error_command_not_found() -> None:
+    assert detect_error_in_output("bash: foo: command not found") is True
+
+
+def test_detect_error_no_such_file() -> None:
+    assert detect_error_in_output("ls: /nope: No such file or directory") is True
+
+
+def test_detect_error_permission_denied() -> None:
+    assert detect_error_in_output("cat: /etc/shadow: Permission denied") is True
+
+
+def test_detect_error_cannot_access() -> None:
+    assert detect_error_in_output("ls: cannot access '/x': No such file") is True
+
+
+def test_detect_error_clean_output() -> None:
+    assert detect_error_in_output("total 12\ndrwxr-xr-x  3 user user 4096 May  3 12:00 .") is False
+
+
+# ── Command.errored / output_bytes wired through build_session_context ──────
+
+
+def test_command_clean_output_not_errored() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "file1\nfile2\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-clean", source="test")
+    assert len(ctx.commands) == 1
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[0].output_bytes == len("file1\nfile2\n")
+
+
+def test_command_with_error_pattern_marked_errored() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("foo\r"),
+        (0.20, "o", "bash: foo: command not found\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-err", source="test")
+    assert ctx.commands[0].errored is True
+    assert ctx.commands[0].output_bytes == len("bash: foo: command not found\n")
+
+
+def test_command_with_ansi_wrapped_error_marked_errored() -> None:
+    """ANSI strip must run before pattern match (red-coloured `Permission denied`)."""
+    events: list[AsciinemaEvent] = [
+        *_typed("cat /etc/shadow\r"),
+        (1.50, "o", "\x1b[31mcat: /etc/shadow: Permission denied\x1b[0m\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-ansi", source="test")
+    assert ctx.commands[0].errored is True
+
+
+def test_last_command_output_window_extends_to_t_end() -> None:
+    """The last command's window has no ``commands[i+1]`` — it spans to t_end."""
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r", t0=0.0),
+        *_typed("foo\r", t0=1.0),
+        (1.50, "o", "bash: foo: command not found\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-last", source="test")
+    assert len(ctx.commands) == 2
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[1].errored is True
+
+
+def test_no_output_events_no_errored() -> None:
+    """A shard with no ``'o'`` events emits clean ``errored=False`` per command."""
+    events: list[AsciinemaEvent] = _typed("ls\r")
+    ctx = build_session_context(events, sid="d0-noout", source="test")
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[0].output_bytes == 0
+
+
+# ── PII regression ──────────────────────────────────────────────────────────
+
+
+def test_pii_no_output_bodies_in_observations() -> None:
+    """Output bytes containing operator-identifying strings must not leak.
+
+    The error pattern triggers ``errored=True``; the surrounding output
+    contains the literal ``secret_payload_xyz`` token. No observation may
+    serialise that token, since the engine only retains a bool + an int.
+    """
+    events: list[AsciinemaEvent] = [
+        *_typed("foo\r"),
+        (0.20, "o", "secret_payload_xyz\nbash: foo: command not found\n"),
+    ]
+    out = list(extract_session(events, sid="d0-pii"))
+    for obs in out:
+        assert "secret_payload_xyz" not in obs.model_dump_json()