feat: session goals - server-driven goal loop with independent small-model audit (#2148)

Arm the target button in the composer and the next prompt becomes a goal:
the server keeps the session working toward it (idle tick -> small-model
audit -> continuation) until the objective is verifiably complete, blocked,
or out of budget — even with the UI closed.

Server (packages/web/server/lib/session-goal):
- event-driven loop on the global SSE hub; goal state lives in
  session.metadata.openchamber.goal (merge-safe patches, stale-write guard
  by goal id), so it survives restarts and syncs to every client for free
- the small-model audit (objective + last assistant turn only, language
  pinned to the objective) is the sole termination authority; blocked needs
  3 consecutive verdicts, audit outages tolerate one unaudited continuation
  then stop the goal as resumable-blocked
- hard stops: optional token budget, auto-continuation cap (Resume grants a
  fresh allowance), turn errors; user abort pauses the goal instead of
  blocking it, and resuming over an aborted tail nudges immediately
- token accounting as a snapshot of the latest turn (input + cache.read +
  output), goal-relative via a creation baseline and segmented across
  compactions; a compaction summary skips the audit and continues
- continuations reuse the session's own provider/model/agent/variant

UI:
- three-mode target button (arm / disarm / manage dialog), informational
  goal strip with inline pause/resume and an Evaluating indicator, sidebar
  state glyph, objective length counter (2000-char server clamp),
  read-only completed goals
- goal entry points: composer (sessions and drafts), start-new-session-
  from-answer dialog, plan implement dialog (plan content becomes the
  objective), scheduled tasks (Run as goal + budget)
- Settings -> Chat -> Goal: feature toggle + default token budget with
  three-layer parity (web server, client persistence, VS Code bridge);
  VS Code renders goal state but hides the entry points (the loop runs in
  the web server only)

Notifications: per-turn "ready" notifications are suppressed while a goal
is active; settling sends one final notification (desktop, web-push, APNs
generic titles with the session name as body) honoring the completion
toggle. Error/question/permission notifications are untouched.

Docs: user guide (session-goals) in all 9 locales + sidebar entry,
scheduled-tasks cross-reference, server module DOCUMENTATION.md.

packages/ui/src/lib/i18n/messages/zh-TW.settings.ts

@@ -1638,6 +1638,12 @@
   'settings.openchamber.visual.field.sessionRecapAria': '代理完成後產生回顧',
   'settings.openchamber.visual.field.sessionSuggestion': '產生下一則使用者訊息建議',
   'settings.openchamber.visual.field.sessionSuggestionAria': '代理完成後產生下一則使用者訊息建議',
+  'settings.openchamber.visual.field.sessionGoal': '啟用工作階段目標',
+  'settings.openchamber.visual.field.sessionGoalAria': '讓工作階段自動朝目標持續工作',
+  'settings.openchamber.visual.goal.sectionTitle': '目標',
+  'settings.openchamber.visual.goal.budgetLabel': '預設權杖預算',
+  'settings.openchamber.visual.goal.budgetAria': '為新目標套用預設權杖預算',
+  'settings.openchamber.visual.goal.description': '在輸入框中啟用靶心按鈕，下一則訊息即成為目標：代理將在小型模型的稽核下自動持續工作，即使你不在電腦前。',
   'settings.openchamber.visual.field.showReasoningTracesAria': '顯示推理軌跡',
   'settings.openchamber.visual.field.showReasoningTraces': '顯示推理軌跡',
   'settings.openchamber.visual.field.collapsibleThinkingBlocksAria': '啟用可摺疊推理區塊',