fix(tools): clarify kanban_complete phantom-card retry guidance

When kanban_complete rejects a created_cards list as hallucinated, the
task is intentionally left in-flight (the gate runs before the write
txn) so the worker can retry with a corrected list or pass
created_cards=[] to skip the check. The retry path already worked, but
the previous error wording read like a terminal failure and workers
were observed abandoning the run instead of trying again.

Spell out the recovery path explicitly in the tool_error response
("Your task is still in-flight ... Retry kanban_complete with ...") and
add regression coverage at both the kernel and tool layers so the
retry contract — and the wording the worker depends on to discover
it — is pinned.

Fixes #22923

tools/kanban_tools.py

@@ -406,12 +406,21 @@ def _handle_complete(args: dict, **kw) -> str:
                 # Structured rejection — surface the phantom ids so the
                 # worker can retry with a corrected list or drop the
                 # field. Audit event already landed in the DB.
+                #
+                # The task itself was NOT mutated (the gate runs before
+                # the write txn), so the worker can simply call
+                # kanban_complete again. Spell that out — without it the
+                # model often interprets a tool_error as a terminal
+                # failure and either blocks or crashes the run instead
+                # of retrying. See #22923.
                 return tool_error(
                     f"kanban_complete blocked: the following created_cards "
                     f"do not exist or were not created by this worker: "
                     f"{', '.join(hall_err.phantom)}. "
-                    f"Either omit them, use only ids returned from successful "
-                    f"kanban_create calls, or remove the created_cards field."
+                    f"Your task is still in-flight (no state change). "
+                    f"Retry kanban_complete with the same summary/metadata "
+                    f"and either drop these ids from created_cards, or pass "
+                    f"created_cards=[] to skip the card-claim check entirely."
                 )
             if not ok:
                 return tool_error(