feat: add managed system prompt optimization

Add an opt-in OpenCode plugin that replaces the built-in provider behavioral prompt with a minimal identity while preserving environment, project, MCP, skill, history, and tool context.

Track the active agent per session and apply the transform only to build and plan. Keep plan/build mode reminders and permission enforcement owned by OpenCode, leave all other agents untouched, and fail safely when the expected prompt boundary is absent.

Expose the feature in Behavior settings with localized guidance, explicit Save + Reload application, settings search integration, persisted boolean validation, and managed-runtime lifecycle composition that does not load the plugin while disabled or on external OpenCode servers.

Document the runtime contract and cover plugin materialization, config preservation, build/plan selection, agent switching, unknown prompt formats, and settings sanitization.

packages/web/server/lib/opencode/DOCUMENTATION.md

@@ -29,6 +29,7 @@ This module provides OpenCode server integration utilities for the web server ru
 - `packages/web/server/lib/opencode/tunnel-wiring-runtime.js`: tunnel service/routes composition runtime and active-port wiring for main server startup.
 - `packages/web/server/lib/opencode/startup-pipeline-runtime.js`: server startup tail orchestration runtime for terminal/proxy/static/start-listen flow.
 - `packages/web/server/lib/agent-tool/runtime.js`: managed OpenCode custom-tool materialization, environment injection, loopback authentication, and fixed CLI action dispatch.
+- `packages/web/server/lib/system-prompt/runtime.js`: opt-in managed OpenCode system-prompt optimizer materialization and plugin injection.
 - `packages/web/server/lib/opencode/server-utils-runtime.js`: shared server runtime utilities for OpenCode proxy wiring, OpenCode port/readiness helpers, and snapshot fetchers.
 - `packages/web/server/lib/opencode/openchamber-routes.js`: OpenChamber update and models metadata route registration.
 - `packages/web/server/lib/opencode/pwa-manifest-routes.js`: PWA manifest route registration with recent-session shortcut resolution and short-lived caching.

packages/web/server/lib/opencode/settings-helpers.js

@@ -494,6 +494,9 @@ export const createSettingsHelpers = (dependencies) => {
     if (typeof candidate.agentControlToolEnabled === 'boolean') {
       result.agentControlToolEnabled = candidate.agentControlToolEnabled;
     }
+    if (typeof candidate.optimizeSystemPrompt === 'boolean') {
+      result.optimizeSystemPrompt = candidate.optimizeSystemPrompt;
+    }
     if (typeof candidate.openCodeUpdateToastDismissedVersion === 'string') {
       const version = candidate.openCodeUpdateToastDismissedVersion.trim();
       result.openCodeUpdateToastDismissedVersion = version.slice(0, VERSION_STRING_MAX_LENGTH);

packages/web/server/lib/opencode/settings-helpers.test.js

@@ -409,6 +409,14 @@ describe('settings helpers', () => {
       expect(helpers.sanitizeSettingsUpdate({ recentEfforts: { 'anthropic/claude-opus-4': [123, ''] } })).toEqual({});
     });
 
+    it('persists only boolean system prompt optimization values', () => {
+      const helpers = createTestHelpersWithRealSanitizers();
+
+      expect(helpers.sanitizeSettingsUpdate({ optimizeSystemPrompt: true })).toEqual({ optimizeSystemPrompt: true });
+      expect(helpers.sanitizeSettingsUpdate({ optimizeSystemPrompt: false })).toEqual({ optimizeSystemPrompt: false });
+      expect(helpers.sanitizeSettingsUpdate({ optimizeSystemPrompt: 'true' })).toEqual({});
+    });
+
     it('survives a full settings.json payload containing all four previously-dropped fields (regression)', () => {
       const helpers = createTestHelpersWithRealSanitizers();
       const payload = {