fix(behave_shell/F.0): tighten prompt detector — log lines ending in '>' no longer vote

_detect_prompt_suffix accepted ANY line ending in $#%> as a PS1 prompt,
so a single `cat /var/log/dpkg.log` (195 lines closing in `<none>`)
flooded environmental.shell_type votes and flipped a plainly-bash
session to fish.

A prompt line now requires either a trailing space after the suffix
(default PS1 shape across bash/zsh/fish/PowerShell) or a PS1-shape
token (user@host, "PS " prefix, or a Windows drive-letter prefix).

Regression tests pin the dpkg.log false-positive and a $-terminated
prose line.

decnet/profiler/behave_shell/_parse.py

@@ -160,20 +160,50 @@ _PROMPT_LINE_RE = re.compile(
 )
 
 
+_PS1_SHAPE_RE = re.compile(
+    # A line that LOOKS like a PS1 prompt carries at least one of:
+    #   - user@host                       (bash/zsh/fish defaults)
+    #   - "PS " prefix                    (PowerShell)
+    #   - drive-letter prefix "C:\" / "C:/"  (cmd.exe / PowerShell)
+    # These tokens are extremely rare in the body of generic command
+    # output, so they discriminate prompts from log lines that
+    # incidentally end with one of $#%>.
+    r"(?:[\w.-]+@[\w.-]+|^\s*PS\s|[A-Za-z]:[\\/])"
+)
+
+
 def _detect_prompt_suffix(line: str) -> str | None:
     """Return the suffix character if ``line`` looks like a PS1 prompt.
 
     ``line`` is one logical output line, ANSI-stripped, trailing
-    whitespace included. The discriminating shape: any text ending in
-    one of ``$ # % >`` optionally followed by a single space. We require
-    the line to be non-empty and the suffix to be the rightmost
-    non-whitespace character.
+    whitespace included. The discriminating shape: a line ending in
+    one of ``$ # % >``, AND either
+
+    * the original line ends with the suffix followed by a trailing
+      space (the default PS1 shape across bash / zsh / fish /
+      PowerShell — ``$ ``, ``# ``, ``% ``, ``> ``), OR
+    * the line carries a recognisable PS1-shape token (``user@host``,
+      ``PS `` prefix, or a Windows drive-letter prefix).
+
+    Without this guard, command output that incidentally ends in one
+    of the suffix characters — e.g. ``dpkg.log`` lines that close with
+    ``<none>`` — was being voted into the shell-type mode and could
+    flip the result to ``fish`` for an obvious-bash session.
     """
     stripped = line.rstrip()
     if not stripped:
         return None
     last = stripped[-1]
-    return last if last in ("$", "#", "%", ">") else None
+    if last not in ("$", "#", "%", ">"):
+        return None
+    # Prefer the cheap structural check: was there a trailing space
+    # after the suffix in the original line? (Default PS1s all carry
+    # one.) Falls back to a PS1-shape token search.
+    if len(line) > len(stripped) and line[len(stripped)] == " ":
+        return last
+    if _PS1_SHAPE_RE.search(stripped):
+        return last
+    return None
 
 
 def extract_prompt_lines(

tests/profiler/behave_shell/test_prompt_line_detection.py

@@ -66,6 +66,32 @@ def test_clean_output_no_prompt() -> None:
     assert lines == []
 
 
+def test_log_lines_ending_in_gt_are_not_prompts() -> None:
+    """``dpkg.log`` style lines close with ``<none>`` — incidentally
+    ending in ``>``. They must NOT register as fish prompts; otherwise
+    a single ``cat /var/log/dpkg.log`` would flood ``shell_type`` votes
+    and flip the mode for a plainly-bash session.
+    """
+    text = (
+        "2026-05-09 02:18:09 configure libssl3:amd64 3.0.19-1~deb12u2 <none>\n"
+        "2026-05-09 02:18:09 configure libexpat1:amd64 2.5.0-1+deb12u2 <none>\n"
+        "2026-05-09 02:18:10 configure python3.11-minimal:amd64 3.11.2-6 <none>\n"
+        "root@host:~# "
+    )
+    lines = list(extract_prompt_lines(text, base_ts=10.0, max_chars=256))
+    assert len(lines) == 1
+    assert lines[0].suffix_char == "#"
+
+
+def test_output_line_ending_in_dollar_without_ps1_shape_rejected() -> None:
+    """Sentence that happens to end in ``$`` (e.g. shell variable in
+    a doc) without trailing space and without a PS1 shape token must
+    not be treated as a prompt."""
+    text = "use $PATH or $HOME\nset -- $\n"
+    lines = list(extract_prompt_lines(text, base_ts=11.0, max_chars=256))
+    assert lines == []
+
+
 def test_long_prompt_capped_to_max_chars() -> None:
     long = "x" * 500 + "$ "
     lines = list(extract_prompt_lines(long, base_ts=6.0, max_chars=256))