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/SessionGoalDialog.tsx

@@ -10,7 +10,7 @@ import { Textarea } from '@/components/ui/textarea';
 import { NumberInput } from '@/components/ui/number-input';
 import { toast } from '@/components/ui';
 import { Checkbox } from '@/components/ui/checkbox';
-import { useSessionGoal } from '@/hooks/useSessionGoal';
+import { useGoalObjectiveContent, useSessionGoal } from '@/hooks/useSessionGoal';
 import {
   formatGoalTokens,
   SESSION_GOAL_OBJECTIVE_CHAR_LIMIT,
@@ -32,6 +32,7 @@ interface SessionGoalDialogProps {
 export function SessionGoalDialog({ open, onOpenChange, sessionId, directory }: SessionGoalDialogProps) {
   const { t } = useI18n();
   const { goal } = useSessionGoal(sessionId, directory);
+  const objectiveContent = useGoalObjectiveContent(sessionId, goal);
 
   const [objective, setObjective] = React.useState('');
   const [budgetEnabled, setBudgetEnabled] = React.useState(false);
@@ -40,7 +41,7 @@ export function SessionGoalDialog({ open, onOpenChange, sessionId, directory }:
 
   React.useEffect(() => {
     if (!open) return;
-    setObjective(goal?.objective ?? '');
+    setObjective(goal?.objectiveFile ? (objectiveContent ?? '') : (goal?.objective ?? ''));
     setBudgetEnabled(Boolean(goal?.tokenBudget));
     setTokenBudget(goal?.tokenBudget ?? 200_000);
     // Seed the form only when the dialog opens; live goal updates while it is
@@ -48,6 +49,14 @@ export function SessionGoalDialog({ open, onOpenChange, sessionId, directory }:
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [open]);
 
+  // File-backed objectives fetch async — the content usually lands right
+  // after the dialog opens. Late-seed the textarea only while it is still
+  // untouched so a slow fetch never clobbers the user's typing.
+  React.useEffect(() => {
+    if (!open || !goal?.objectiveFile || objectiveContent === null) return;
+    setObjective((current) => (current === '' ? objectiveContent : current));
+  }, [open, goal?.objectiveFile, objectiveContent]);
+
   const run = React.useCallback(async (action: () => Promise<void>, closeAfter: boolean) => {
     setBusy(true);
     try {
@@ -62,7 +71,8 @@ export function SessionGoalDialog({ open, onOpenChange, sessionId, directory }:
   }, [onOpenChange, t]);
 
   const trimmedObjective = objective.trim();
-  const objectiveChanged = trimmedObjective !== (goal?.objective ?? '');
+  const savedObjective = goal?.objectiveFile ? (objectiveContent ?? '') : (goal?.objective ?? '');
+  const objectiveChanged = trimmedObjective !== savedObjective;
   const budgetValue = budgetEnabled ? tokenBudget : null;
   const budgetChanged = budgetValue !== (goal?.tokenBudget ?? null);
   // A completed goal is read-only: remove it and arm a new one instead of
@@ -112,7 +122,7 @@ export function SessionGoalDialog({ open, onOpenChange, sessionId, directory }:
           )}
 
           {isCompleted ? (
-            <p className="max-h-48 overflow-y-auto whitespace-pre-wrap break-words typography-meta text-muted-foreground">{goal.objective}</p>
+            <p className="max-h-48 overflow-y-auto whitespace-pre-wrap break-words typography-meta text-muted-foreground">{objectiveContent ?? goal.objective}</p>
           ) : (
             <>
               <div className="space-y-1">