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/web/server/lib/scheduled-tasks/runtime.js

@@ -444,9 +444,22 @@ export const createScheduledTasksRuntime = (deps) => {
   // from session events like any other goal.
   const createTaskGoal = async ({ baseUrl, authHeaders, sessionID, projectPath, task }) => {
     const now = Date.now();
+    // File-backed objective keyed by session id: metadata stays light, the
+    // full expanded prompt lives under the OpenChamber data dir. If the file
+    // write fails, fall back to an inline (clamped) objective.
+    const objectiveText = expandSnippets(task.execution.prompt, projectPath);
+    let objectiveFile = false;
+    try {
+      const { writeObjective } = await import('../session-goal/objectives.js');
+      await writeObjective(sessionID, objectiveText);
+      objectiveFile = true;
+    } catch (error) {
+      console.warn('[scheduled-tasks] goal objective file write failed, falling back to inline:', error?.message || error);
+    }
     const goal = {
       id: `${now.toString(36)}${Math.random().toString(36).slice(2, 8)}`,
-      objective: expandSnippets(task.execution.prompt, projectPath).slice(0, 2000),
+      objective: objectiveFile ? '' : objectiveText.slice(0, 5000),
+      objectiveFile,
       status: 'active',
       tokenBudget: task.execution.goalTokenBudget || null,
       tokensUsed: 0,