feat(webui): add assistant reply fork-from-here

nanobot/session/manager.py

@@ -5,6 +5,7 @@ import os
 import re
 import shutil
 from contextlib import suppress
+from copy import deepcopy
 from dataclasses import dataclass, field
 from datetime import datetime
 from pathlib import Path
@@ -30,6 +31,14 @@ _TOOL_CALL_ECHO_RE = re.compile(r'^\s*(?:generate_image|message)\([^)]*\)\s*$')
 _SESSION_PREVIEW_MAX_CHARS = 120
 _SESSION_LIST_PREVIEW_MAX_RECORDS = 200
 _SESSION_LIST_PREVIEW_MAX_CHARS = 1_000_000
+_FORK_VOLATILE_METADATA_KEYS = {
+    "goal_state",
+    "pending_user_turn",
+    "runtime_checkpoint",
+    "thread_goal",
+    "title",
+    "title_user_edited",
+}
 
 
 def _sanitize_assistant_replay_text(content: str) -> str:
@@ -628,6 +637,62 @@ class SessionManager:
             logger.warning("Failed to delete session file {}: {}", path, e)
             return False
 
+    def fork_session_before_user_index(
+        self,
+        source_key: str,
+        target_key: str,
+        before_user_index: int,
+    ) -> Session | None:
+        """Create *target_key* from *source_key* before a global user-message index.
+
+        ``before_user_index`` is zero-based over user messages in the full session:
+        ``0`` means "before the first user message", ``1`` means "before the
+        second user message", and so on. A value equal to the total user-message
+        count copies the full session prefix. The target user message itself is
+        not copied; the WebUI pre-fills it in the composer for editing and resend.
+        """
+        if before_user_index < 0:
+            return None
+        source = self._cache.get(source_key) or self._load(source_key)
+        if source is None:
+            return None
+
+        copied: list[dict[str, Any]] = []
+        user_index = 0
+        found_target = False
+        for message in source.messages:
+            if message.get("role") == "user":
+                if user_index == before_user_index:
+                    found_target = True
+                    break
+                user_index += 1
+            copied.append(deepcopy(message))
+        if user_index == before_user_index:
+            found_target = True
+        if not found_target:
+            return None
+
+        metadata = deepcopy(source.metadata)
+        for key in _FORK_VOLATILE_METADATA_KEYS:
+            metadata.pop(key, None)
+
+        last_consolidated = min(source.last_consolidated, len(copied))
+        if source.last_consolidated > len(copied):
+            metadata.pop("_last_summary", None)
+            last_consolidated = 0
+
+        now = datetime.now()
+        target = Session(
+            key=target_key,
+            messages=copied,
+            created_at=now,
+            updated_at=now,
+            metadata=metadata,
+            last_consolidated=last_consolidated,
+        )
+        self.save(target, fsync=True)
+        return target
+
     def read_session_file(self, key: str) -> dict[str, Any] | None:
         """Load a session from disk without caching; intended for read-only HTTP endpoints.