feat: file-backed goal objectives + compaction accounting fixes

Compaction fixes (observed in a real long run):
- the summary message's zeroed tokens froze the goal counter at its
  pre-compaction value; segments now close with the previously displayed
  total as a continuity floor
- audits and continuations after a summary tail now take execution params
  (provider/model/agent/variant) from the newest non-summary assistant
  turn instead of inheriting agent 'compaction' and the summarize model

File-backed objectives:
- the objective text lives in <data-dir>/goals/<sessionId>.md, keyed by
  session id (one goal per session, a new goal overwrites the file);
  metadata carries only an objectiveFile flag so session.updated fanout
  stays light, and never a path — ids are pattern-validated before any
  filesystem access
- limit raised to 5000 chars, no snapshot field: the UI fetches content
  via PUT/GET/DELETE /api/goals/objective/:sessionId (behind the blanket
  /api auth gate), writes the file before stamping metadata, and falls
  back to an inline objective when the write fails
- the loop reads the file fresh on every tick, so objectives are
  live-editable mid-goal; a missing file falls back to the inline text
- scheduled goal tasks write the objective file server-side; VS Code
  degrades to the audit note (route unavailable there by design)

packages/ui/src/components/chat/SessionGoalRow.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import { Icon } from '@/components/icon/Icon';
 import { useSessionStatus } from '@/sync/sync-context';
-import { useSessionGoal } from '@/hooks/useSessionGoal';
+import { useGoalObjectiveContent, useSessionGoal } from '@/hooks/useSessionGoal';
 import { formatGoalTokens } from '@/lib/sessionGoalMetadata';
 import { sessionGoalStatusColor, sessionGoalStatusLabelKey } from '@/lib/sessionGoalPresentation';
 import { setSessionGoalStatus } from '@/lib/sessionGoalActions';
@@ -22,6 +22,7 @@ interface SessionGoalRowProps {
 export const SessionGoalRow: React.FC<SessionGoalRowProps> = React.memo(({ sessionId, directory, className }) => {
   const { t } = useI18n();
   const { goal, enabled } = useSessionGoal(sessionId ?? '', directory);
+  const objectiveContent = useGoalObjectiveContent(sessionId ?? '', goal);
   const sessionStatus = useSessionStatus(sessionId ?? '', directory);
   const [busy, setBusy] = React.useState(false);
 
@@ -65,11 +66,11 @@ export const SessionGoalRow: React.FC<SessionGoalRowProps> = React.memo(({ sessi
         className,
       )}
       aria-label={t('chat.goal.row.aria')}
-      title={goal.objective}
+      title={objectiveContent ?? undefined}
     >
       <Icon name="target" className="h-3.5 w-3.5 flex-shrink-0" style={{ color: sessionGoalStatusColor[goal.status] }} aria-hidden="true" />
       <span className="min-w-0 flex-1 truncate typography-meta text-foreground">
-        {goal.note || goal.objective}
+        {goal.note || objectiveContent || ''}
       </span>
       {goal.status === 'active' && (!sessionStatus || sessionStatus.type === 'idle') ? (
         // The agent stopped but the goal is still active: the server is