fix(agent): pop agent_stack on failed handoff; guard recent_turns<=0 (#60)

Three handoff-path fixes, verified against the merged-onto-new-dev state.

- executor.py: a failed handoff now pops the target whenever it is on top of agent_stack (it was pushed only after early depth/cycle/registry checks passed), regardless of return_to_parent. Prevents a stuck target from tripping false cycle detection on retry and inflating agents_used.
- handoff/history.py: _find_turn_boundary returns len(messages) for recent_turns<=0 so the recent slice is empty (RECENT_N/HYBRID). Previously user_indices[-0] returned nearly the full history.
- utils/sanitization.py: allow '|' in user_id/conversation_id (Auth0-style 'sub' claims). ':' remains the only excluded key delimiter; nothing splits on '|', so no scope-bleed.

Adds adversarial tests for handoff depth/cycle/stack-cleanup/summarization and a whitespace user_id journey test.

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: shyftlabs-satyam

src/continuum/agent/execution/executor.py

@@ -639,10 +639,14 @@ async def _execute_loop_impl(
                                         messages=messages,
                                     )
                             else:
-                                # Handoff failed — clean up stack if return_to_parent
-                                # (push_agent already ran in execute_handoff)
-                                handoff_config = agent.get_handoff(target)
-                                if handoff_config and handoff_config.return_to_parent:
+                                # Handoff failed. execute_handoff pushes the target
+                                # only AFTER its early depth/cycle/registry checks
+                                # pass, so the target is on the stack only in
+                                # post-push failures — where it sits on top. Pop it
+                                # whenever it's on top, regardless of return_to_parent,
+                                # so a failed hop never leaves the target stuck (which
+                                # would trip false cycle detection on a retry).
+                                if run_state.agent_stack and run_state.agent_stack[-1] == target:
                                     run_state.pop_agent()
                                     run_state.current_agent = agent.name
                                 # Add error as tool result so the LLM can react

src/continuum/agent/handoff/history.py

@@ -58,6 +58,12 @@ def _find_turn_boundary(messages: list[dict[str, Any]], n_turns: int) -> int:
     A turn starts at each user message. All messages within a turn
     (tool calls, tool results, assistant responses) are included.
     """
+    # n_turns <= 0 means "keep no recent turns". Return an index past the end
+    # so messages[boundary:] == []. Without this guard, n_turns == 0 hits
+    # user_indices[-0] (== user_indices[0]) and returns almost the entire
+    # history instead of nothing.
+    if n_turns <= 0:
+        return len(messages)
     user_indices = [i for i, m in enumerate(messages) if m.get("role") == "user"]
     if len(user_indices) <= n_turns:
         return 0

src/continuum/utils/sanitization.py

@@ -60,11 +60,13 @@ def sanitize_message_content(message: dict[str, Any]) -> dict[str, Any]:
     return message
 
 
-# Allowlist for user_id and conversation_id: alphanumeric, hyphen, underscore, dot, @
+# Allowlist for user_id and conversation_id: alphanumeric, hyphen, underscore, dot, @, pipe.
 # Colons are explicitly excluded — they are key delimiters in Redis session keys
 # ("c:{conversation_id}:u:{user_id}") and cart_session_id (f"{user_id}:{conversation_id}").
 # A colon in either value would silently collapse into another user's scoping key.
-_ID_ALLOWED_RE = re.compile(r"[^a-zA-Z0-9\-_.@]")
+# Pipe ("|") IS allowed: Auth0-style JWT 'sub' claims look like "auth0|64abc..." and
+# the pipe is not a key delimiter here, so it is safe (PR #55 review follow-up).
+_ID_ALLOWED_RE = re.compile(r"[^a-zA-Z0-9\-_.@|]")
 _ID_MAX_LENGTH = 128
 
 
@@ -112,7 +114,7 @@ def _validate_id(value: str | None, field: str) -> str | None:
             field,
             value,
             f"contains disallowed character {bad.group()!r}; "
-            "allowed characters are letters, digits, and - _ . @",
+            "allowed characters are letters, digits, and - _ . @ |",
         )
     return cleaned