feat(conversations,api,ui): rewind takes the last turns of a thread back, the cut session stays in the store

src/beaver_gateway/backends/transcript.py

@@ -20,13 +20,14 @@ except ImportError:  # pragma: no cover
     _cli_version = "2.1.248"
 
 if TYPE_CHECKING:
-    from collections.abc import Iterable
+    from collections.abc import Iterable, Sequence
 
 __all__ = [
     "CLI_VERSION",
     "build_entries",
     "close_open_tool_uses",
     "fingerprint",
+    "last_message_before",
     "messages_from_entries",
     "open_tool_uses",
     "prompt_count",
@@ -35,6 +36,7 @@ __all__ = [
     "spoken_text",
     "strip_tool_entries",
     "text_of",
+    "turn_starts",
     "window_entries",
 ]
 
@@ -438,6 +440,33 @@ def prompt_count(entries: Iterable[Mapping[str, Any]]) -> int:
     return sum(1 for e in entries if _is_prompt_entry(e))
 
 
+def turn_starts(entries: Iterable[Mapping[str, Any]]) -> list[int]:
+    """Where each turn opens: the index of every prompt the SDK was handed.
+
+    The CLI stamps only those with ``promptSource``; what it writes as a
+    user on its own (an interruption note, a skill body) stays inside the
+    turn it belongs to.
+    """
+    return [
+        index
+        for index, entry in enumerate(entries)
+        if entry.get("promptSource") and _is_prompt_entry(entry)
+    ]
+
+
+def last_message_before(entries: Sequence[Mapping[str, Any]], index: int) -> str | None:
+    """Uuid of the last ``user``/``assistant`` entry ahead of ``index``."""
+    for entry in reversed(entries[:index]):
+        uuid = entry.get("uuid")
+        if (
+            entry.get("type") in _PROMPT_TYPES
+            and isinstance(uuid, str)
+            and not entry.get("isSidechain")
+        ):
+            return uuid
+    return None
+
+
 def spoken_text(block: Any, *, thinking_is_progress: bool) -> str:
     """What a content block (dict or model) says to the human.