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/web/server/lib/projects/project-config.js

@@ -213,6 +213,12 @@ const normalizeExecution = (value) => {
   const modelID = asNonEmptyString(value.modelID);
   const variant = asNonEmptyString(value.variant);
   const agent = asNonEmptyString(value.agent);
+  const goalEnabled = value.goalEnabled === true;
+  const goalTokenBudget = typeof value.goalTokenBudget === 'number'
+    && Number.isFinite(value.goalTokenBudget)
+    && value.goalTokenBudget > 0
+    ? Math.floor(value.goalTokenBudget)
+    : undefined;
 
   if (!prompt) {
     throw new Error('execution.prompt is required');
@@ -230,6 +236,8 @@ const normalizeExecution = (value) => {
     modelID,
     ...(variant ? { variant } : {}),
     ...(agent ? { agent } : {}),
+    ...(goalEnabled ? { goalEnabled: true } : {}),
+    ...(goalEnabled && goalTokenBudget ? { goalTokenBudget } : {}),
   };
 };