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/opencode/core-routes.js

@@ -1000,6 +1000,7 @@ export const registerCommonRequestMiddleware = (app, dependencies) => {
       req.path.startsWith('/api/notifications') ||
       req.path.startsWith('/api/session-folders') ||
       req.path.startsWith('/api/small-model') ||
+      req.path.startsWith('/api/goals') ||
       req.path.startsWith('/api/text') ||
       req.path.startsWith('/api/voice') ||
       req.path.startsWith('/api/tts') ||

packages/web/server/lib/opencode/feature-routes-runtime.js

@@ -1,6 +1,7 @@
 import { registerFsRoutes } from '../fs/routes.js';
 import { registerQuotaRoutes } from '../quota/routes.js';
 import { registerSmallModelRoutes } from '../small-model/routes.js';
+import { registerSessionGoalRoutes } from '../session-goal/routes.js';
 import { registerGitHubRoutes } from '../github/routes.js';
 import { registerGitRoutes } from '../git/routes.js';
 import { registerMagicPromptRoutes } from '../magic-prompts/routes.js';
@@ -236,6 +237,7 @@ export const createFeatureRoutesRuntime = (dependencies) => {
 
     registerQuotaRoutes(app, { getQuotaProviders });
     registerSmallModelRoutes(app, { getSmallModelService });
+    registerSessionGoalRoutes(app);
     registerGitHubRoutes(app);
     registerGitRoutes(app);
     registerMagicPromptRoutes(app, {