fix(history): don't parse tool_result as ItemStartedEvent in deferred dequeue

[Antisophy]

Problem

Opening a task whose history has a steering message immediately followed by a tool_result crashes the server with an uncaught object.Exception (exit 1). ensureHistoryLoaded throws on every click, so the task's history never loads.

Abbreviated stack trace:

object.Exception@ae/utils/serialization/serialization.d(1007): Unknown field tool_result
  ... jsonParse!(cydo.agent.protocol.ItemStartedEvent) ...
  source/cydo/app.d  (ensureHistoryLoaded, deferred-dequeue branch)

Root cause

In the deferred-dequeue branch, after a queue dequeue the code treats the next type:"user" line as the steering echo and parses it strictly as ItemStartedEvent. But when the turn was interrupted mid-tool-use, that line is a tool_result, which translateHistoryLine turns into an item/result event whose tool_result field ItemStartedEvent doesn't declare. ItemStartedEvent is parsed strictly, so the unknown field throws and the exception is uncaught.

Fix

Peek at the translated event's type with a @JSONPartial probe and only run the ItemStartedEvent parse/inject path when it's item/started. Otherwise emit the translated events unchanged and stay deferred for the real echo. ItemStartedEvent is left strict so round-tripping an item/result through it can't silently drop tool_result.

Repro

A Claude history file with these three lines; open the task in the UI:

{"type":"queue-operation","operation":"enqueue","timestamp":"2026-06-11T06:00:00Z","sessionId":"S","content":"are you under control?"}
{"type":"queue-operation","operation":"dequeue","timestamp":"2026-06-11T06:00:01Z","sessionId":"S"}
{"parentUuid":"p","isSidechain":false,"type":"user","message":{"role":"user","content":[{"tool_use_id":"toolu_1","type":"tool_result","content":"ok"}]}}