feat(session): persist optional time zones

docs/subsystems/persistence.md

@@ -38,7 +38,7 @@ interface SessionLocation {
 
 ## `SessionHeader` — metadata beside the log
 
-Per-session metadata travels **separately** from the event log: format version, cwd, lineage, and the seed boundary are storage concerns, not conversation events, so they stay out of `SessionEventMap` and never reach `deriveMessages()`. The header is attached to a `Session` via `session.header`.
+Per-session metadata travels **separately** from the event log: format version, cwd, optional caller-validated time zone, lineage, and the seed boundary are storage concerns, not conversation events, so they stay out of `SessionEventMap` and never reach `deriveMessages()`. The header is attached to a `Session` via `session.header`.
 
 Source: [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts)
 
@@ -59,6 +59,11 @@ interface SessionHeader {
   readonly createdAt: number
   /** Absolute working directory the session was created in (if any). */
   readonly cwd?: string
+  /**
+   * Optional caller-validated time-zone identifier captured at creation.
+   * Session core preserves the exact string without interpreting or canonicalizing it.
+   */
+  readonly timeZone?: string
   /** The session this one was forked from (seed lineage), if any. */
   readonly parentSession?: SessionId
   /**
@@ -82,7 +87,7 @@ interface SessionHeader {
 
 ## `CreateSessionOptions` — seeding and metadata
 
-Creating a `Session` through the store takes a `seed` (initial replay or fork history) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller may supply the validated absolute `cwd`, the `parentSession` lineage, the `seedLength` seed boundary, the optional coarse `origin`, the `delegationDepth`, and an existing `createdAt`. `origin: 'subagent'` lets product navigation hide duplicate child rows; it does not prove that a descriptor is valid or that the child can resume.
+Creating a `Session` through the store takes a `seed` (initial replay or fork history) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller supplies the validated absolute `cwd`, optional caller-validated `timeZone`, the `parentSession` lineage, the `seedLength` seed boundary, the optional coarse `origin`, the `delegationDepth`, and — only when reconstructing a persisted session — the original `createdAt` to preserve it. `origin: 'subagent'` lets product navigation hide duplicate child rows; it does not prove that a descriptor is valid or that the child can resume.
 
 ```ts type-equiv
 /**
@@ -99,6 +104,8 @@ interface CreateSessionOptions {
    */
   readonly meta?: {
     readonly cwd?: string
+    /** Caller-validated time-zone identifier to preserve verbatim in the header. */
+    readonly timeZone?: string
     readonly parentSession?: SessionId
     readonly createdAt?: number
     readonly seedLength?: number

docs/subsystems/persistence.zh.md

@@ -38,7 +38,7 @@ interface SessionLocation {
 
 ## `SessionHeader`：日志旁的元数据
 
-每个会话的元数据与事件日志**分开**存储：格式版本、cwd、血统与 seed 边界是存储层关注点而非对话事件，因此不进入 `SessionEventMap`，也不会到达 `deriveMessages()`。header 通过 `session.header` 附加到 `Session` 上。
+每个会话的元数据与事件日志**分开**存储：格式版本、cwd、可选且由调用方校验的时区、血统与 seed 边界是存储层关注点而非对话事件，因此不进入 `SessionEventMap`，也不会到达 `deriveMessages()`。header 通过 `session.header` 附加到 `Session` 上。
 
 源码：[`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts)
 
@@ -59,6 +59,11 @@ interface SessionHeader {
   readonly createdAt: number
   /** Absolute working directory the session was created in (if any). */
   readonly cwd?: string
+  /**
+   * Optional caller-validated time-zone identifier captured at creation.
+   * Session core preserves the exact string without interpreting or canonicalizing it.
+   */
+  readonly timeZone?: string
   /** The session this one was forked from (seed lineage), if any. */
   readonly parentSession?: SessionId
   /**
@@ -82,7 +87,7 @@ interface SessionHeader {
 
 ## `CreateSessionOptions`：seed 与元数据
 
-通过 store 创建 `Session` 时会接收 `seed`（初始回放或 fork 历史）与 `meta`（store 折叠进 `SessionHeader` 的存储层字段）。store 填充 `version`/`id` 并为 `createdAt` 提供默认值；调用方可以提供已校验的绝对 `cwd`、`parentSession` 谱系、`seedLength` 种子边界、可选的粗粒度 `origin`、`delegationDepth` 以及已有的 `createdAt`。`origin: 'subagent'` 让产品导航能够隐藏重复的 child 行；它不证明描述符有效，也不证明 child 可以恢复。
+通过 store 创建 `Session` 时会接收 `seed`（初始回放或 fork 历史）与 `meta`（store 折叠进 `SessionHeader` 的存储层字段）。store 填充 `version`/`id` 并为 `createdAt` 提供默认值；调用方提供已校验的绝对 `cwd`、可选且由调用方校验的 `timeZone`、`parentSession` 谱系、`seedLength` 种子边界、可选的粗粒度 `origin`、`delegationDepth`，以及——仅在重建已持久化会话时——需要保留的原始 `createdAt`。`origin: 'subagent'` 让产品导航能够隐藏重复的 child 行；它不证明描述符有效，也不证明 child 可以恢复。
 
 ```ts type-equiv
 /**
@@ -99,6 +104,8 @@ interface CreateSessionOptions {
    */
   readonly meta?: {
     readonly cwd?: string
+    /** Caller-validated time-zone identifier to preserve verbatim in the header. */
+    readonly timeZone?: string
     readonly parentSession?: SessionId
     readonly createdAt?: number
     readonly seedLength?: number

docs/subsystems/session.md

@@ -359,8 +359,8 @@ declare class Session {
   /** The ordered surface over this session's event log. */
   get surface(): SessionSurface;
   /**
-   * Detached, deep-frozen creation metadata (format version, cwd, lineage,
-   * seed boundary). Supplied by the store via `ctx.sessions.create()`. When a
+   * Detached, deep-frozen creation metadata (format version, cwd, time zone,
+   * lineage, seed boundary). Supplied by the store via `ctx.sessions.create()`. When a
    * `Session` is created without a store-owned header, a minimal header is
    * synthesized (stamped with the current {@link SESSION_FORMAT_VERSION}) so
    * `session.header` is always present. Kept out of the event log — it is a

docs/subsystems/session.zh.md

@@ -361,8 +361,8 @@ declare class Session {
   /** The ordered surface over this session's event log. */
   get surface(): SessionSurface;
   /**
-   * Detached, deep-frozen creation metadata (format version, cwd, lineage,
-   * seed boundary). Supplied by the store via `ctx.sessions.create()`. When a
+   * Detached, deep-frozen creation metadata (format version, cwd, time zone,
+   * lineage, seed boundary). Supplied by the store via `ctx.sessions.create()`. When a
    * `Session` is created without a store-owned header, a minimal header is
    * synthesized (stamped with the current {@link SESSION_FORMAT_VERSION}) so
    * `session.header` is always present. Kept out of the event log — it is a