docs(plan): add chat state machine refactor design

Update implementation_plan.md with a detailed refactor plan that splits
`chatInitActor` into two independent actors (`loadQuotaActor` +
`loadHistoryActor`) and redesigns the history loading flow to follow
local → network → save semantics so the UI sees local data first,
then network data.

Key changes outlined in the plan:
- Add new events: `ChatQuotaLoaded`, `ChatHistoryLocalLoaded`,
  `ChatHistoryNetworkLoaded`, `ChatHistorySyncDone`
- Remove dead-code `ChatInit` event
- Extend `ChatState` with `quotaLoaded` and `historyLoaded` flags
- Use an `always` barrier in `guestSession.initializing` and
  `userSession.initializing` so both tasks must complete before
  transitioning to `ready`
- Guest init runs `loadQuota` + `loadHistory` in parallel; non-guest
  init runs `chatWebSocket` + `loadHistory` as independent tasks

Also adds `implementation_plan` to .gitignore.

src/stores/chat/chat-machine.helpers.ts

@@ -96,14 +96,6 @@ export function mapTotalQuotaResult(
   return fallback;
 }
 
-// ============================================================
-// Init 任务 1：游客配额（**纯** local）—— `loadQuotaActor` 调
-// ============================================================
-/**
- * 读取游客配额（**仅** guestSession 有意义）
- * - 游客**日**配 + 总**配** 两**项**都**从** ChatStorage 拉（**纯** local，**无**网络）
- * - 失**败**返 fallback 0（**不**抛）—— **让** UI **还**是**能**进** ready
- */
 export async function readGuestQuota(): Promise<{
   remaining: number;
   total: number;