fix: fork subagents recursed forever in the Claude Code export

A fork subagent's transcript opens with a copy of the parent turn that spawned it, its own Agent call included; folding that call re-entered the same fork until RecursionError, failing every daily sync since 2026-09-24. Skip a spawn call while its own subagent is being expanded, and strip the <fork-boilerplate> preamble.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LbnmGHnFqDjyhPcCg1SEfF

src/providers/claude_code.py

@@ -118,7 +118,10 @@ _HARNESS_TAG_RE = re.compile(
     r"|<command-args>.*?</command-args>"
     r"|<command-contents>.*?</command-contents>"
     r"|<local-command-stdout>.*?</local-command-stdout>"
-    r"|<system-reminder>.*?</system-reminder>",
+    r"|<system-reminder>.*?</system-reminder>"
+    # A fork subagent's first user turn: generic worker rules ahead of the
+    # fork's actual "Your directive: …", which is kept.
+    r"|<fork-boilerplate>.*?</fork-boilerplate>",
     re.DOTALL,
 )
 
@@ -530,6 +533,7 @@ def _extract_messages(
     policy: str,
     subagents: dict | None = None,
     include_sidechain: bool = False,
+    expanding: frozenset[str] = frozenset(),
 ) -> list[dict]:
     """Normalize Claude Code records into messages.
 
@@ -540,6 +544,13 @@ def _extract_messages(
     those recursive subagent passes (subagent records are flagged
     ``isSidechain``); the top-level pass keeps skipping sidechain records so a
     subagent is never also emitted as a stray top-level turn.
+
+    ``expanding`` holds the spawn ids of the subagents being folded around
+    this pass. A ``fork`` subagent's transcript opens with a copy of the parent
+    turn that spawned it, its own spawn call included; that copy is dropped,
+    since the enclosing subagent block already stands for it. Expanding it
+    again recursed without end (RecursionError, every daily sync from
+    2026-09-24).
     """
     subagents = subagents or {}
     messages: list[dict] = []
@@ -620,6 +631,8 @@ def _extract_messages(
                 elif item_type == "tool_use":
                     name = item.get("name") or "tool"
                     tool_id = item.get("id")
+                    if tool_id in expanding:
+                        continue
                     if name in _SUBAGENT_TOOL_NAMES and tool_id in subagents:
                         # A Task/Agent spawn: fold its separate transcript inline
                         # instead of collapsing it. Its own tool traffic is
@@ -633,6 +646,7 @@ def _extract_messages(
                             policy,
                             subagents,
                             include_sidechain=True,
+                            expanding=expanding | {tool_id},
                         )
                         blocks.append(
                             make_subagent_block(