feat: 权限档位体系（三档 plan/workspace/full + 四桥 from_session_id）

L2 核心改动：sessions 表补 permission_mode / permission_enforcement 两列
（sqlite + pg 同步），三桥 lib/permission-mode.js 翻译档位到平台原生配置，
homeagent advisory 模式提示词告知模型实际强制力。四桥全部携带 from_session_id
供 relay 去重与会话回溯。

FromHuman / ToHuman 判据已加入心跳 payload 与 notify/mail.go。

plugins/dsh-mail-bridge/lib/permission-mode.d.ts

@@ -0,0 +1,33 @@
+export const MODE_PLAN: 'plan';
+export const MODE_WORKSPACE: 'workspace';
+export const MODE_FULL: 'full';
+export const MODES: string[];
+export const DEFAULT_MODE: string;
+
+export const ENFORCE_NATIVE: 'native';
+export const ENFORCE_ADVISORY: 'advisory';
+
+export function normalizeMode(mode: unknown): string;
+export function normalizeEnforcement(e: unknown): string;
+export function modeAtMost(a: string, b: string): string;
+export function modeNeedsHuman(mode: string): boolean;
+
+export interface OpencodePermissionRule {
+  permission: string;
+  action: string;
+  pattern: string;
+}
+export function opencodePermissions(mode: string): OpencodePermissionRule[];
+
+export function dshSandboxMode(mode: string): 'read-only' | 'workspace-write' | 'danger-full-access';
+export function dshApprovalPolicy(mode: string): 'ask' | 'never';
+
+export function piGuardedTools(mode: string): string[];
+export function piBlocksOutright(mode: string): boolean;
+
+export interface ModeBriefingCtx {
+  mode: string;
+  enforcement: string;
+  workspace?: string;
+}
+export function modeBriefing(ctx: ModeBriefingCtx): string;