docs(spec): session.positions 不再教一个 hook 里跑不通的调用 (#6001)

`positions` 的边界结论不变 —— 描述性、非授权输入。变的是它旁边那个例子,
因为例子本身是缺陷形状:

FROM「forwarding it as the sharing service's evaluation context
(`services.sharing.canEdit(..., { positions })`,两页 runtime-services
文档都这么教)」
TO 一个 hook 真拿得到的通道 —— 定制消息、经 `ctx.api` 跑业务判断、日志 ——
并写明 hook 上下文没有 `services` 键。

#5720 已实测钉死:hook 上下文逐键构造,ObjectQL 的 buildSession() 与
buildSandboxContext()(runtime/src/sandbox/body-runner.ts —— input /
previous / user / session / event / object / result / api / log / crypto)
都不设 `services`。于是 `services.sharing.canEdit(…)` 在 hook 里是
`undefined()`,外面惯常包的 `if (!ok) throw` 会拒掉每一次写。文档举为正例的
那个形状,恰是对全量流量 fail-close 的那个。

被引用的两页文档在 PR #5938 后已反转(sharing-service.mdx 新增
「Enforcement is automatic — do not re-check it in a hook」;examples.mdx
改教 `ctx.api`),这段 JSDoc 是最后一处还在教已撤回形状的地方。交叉引用改指
那一节。

JSDoc 与 `.describe()` 同改:describe 是 /api/v1/meta/types/hook 对外供给、
Studio 表单渲染的那一份,只改 JSDoc 会把缺陷例子留在作者真正会看的面上。
仅文案,无 schema / 校验 / 运行时行为变化。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011M7UwH25Unfi73UHim7ajY

Author: claude

.changeset/hook-positions-reachable-example.md

@@ -0,0 +1,36 @@
+---
+"@objectstack/spec": patch
+---
+
+docs(spec): `session.positions` stops teaching a hook call that cannot run (#6001)
+
+The `positions` key on the hook session carries a deliberate boundary note: it is
+**descriptive, never an authorization input**. That conclusion is unchanged. What
+changed is the example standing next to it, which was itself defective:
+
+- FROM: "A hook may READ this … forwarding it as the sharing service's evaluation
+  context (`services.sharing.canEdit(..., { positions })`, the shape both
+  `content/docs/kernel/runtime-services/` pages teach)"
+- TO: an example a hook can actually reach — tailoring a message, branching a
+  *business* rule through the hook's own `ctx.api` channel, logging — plus an
+  explicit note that a hook context carries **no `services` key**.
+
+#5720 pinned this by measurement: hook contexts are assembled key by key, and
+neither ObjectQL's `buildSession()` nor `buildSandboxContext()`
+(`packages/runtime/src/sandbox/body-runner.ts` — `input` / `previous` / `user` /
+`session` / `event` / `object` / `result` / `api` / `log` / `crypto`) ever sets
+`services`. So `services.sharing.canEdit(…)` is `undefined()` inside a hook, and
+the customary `if (!ok) throw` wrapped around it rejects **every** write. The
+one shape the doc held up as correct practice was the exact shape that fails
+closed on all traffic.
+
+The two cross-referenced doc pages had already reversed under PR #5938 —
+`sharing-service.mdx` now carries "Enforcement is automatic — do not re-check it
+in a hook", and `examples.mdx` teaches `ctx.api` — leaving this JSDoc as the last
+site still teaching the withdrawn shape. The reference now points at that
+section instead of at the pages generically.
+
+Both the JSDoc **and** the `.describe()` are updated; the `describe` is the copy
+served through `/api/v1/meta/types/hook` and rendered in Studio's form, so
+leaving it would have kept the defective example on the surface authors actually
+read. Text only — no schema, validation, or runtime behaviour change.

packages/spec/src/data/hook.zod.ts

@@ -418,13 +418,25 @@ export const HookContextSchema = lazySchema(() => z.object({
      * `buildSession()` (`packages/objectql/src/engine.ts`).
      *
      * ⚠️ **Descriptive, NOT an authorization input.** A hook may READ this to
-     * describe the caller — forwarding it as the sharing service's evaluation
-     * context (`services.sharing.canEdit(..., { positions })`, the shape both
-     * `content/docs/kernel/runtime-services/` pages teach), tailoring a
-     * message, logging — and nothing more. It grants nothing on its own, no
-     * security middleware keys on it here, and a hook must never make the
-     * access decision itself by testing it
+     * describe the caller — tailoring a message, branching a *business* rule
+     * through its own channel (`ctx.api`), logging — and nothing more. It
+     * grants nothing on its own, no security middleware keys on it here, and a
+     * hook must never make the access decision itself by testing it
      * (`session.positions.includes('sales_manager')` is the anti-pattern).
+     *
+     * The example that used to stand here — forwarding this array as the
+     * sharing service's evaluation context,
+     * `services.sharing.canEdit(..., { positions })` — was itself unreachable
+     * and is gone (#6001). A hook context is assembled key by key by the engine
+     * (`buildSession()` / `buildSandboxContext()` in
+     * `packages/runtime/src/sandbox/body-runner.ts`) and carries **no
+     * `services` key**, so that call is `undefined()` at run time and the
+     * customary `if (!ok) throw` around it rejects every write (#5720). It was
+     * a defect shape wearing a good-practice label. The sharing gates run
+     * inside the engine and have already thrown `FORBIDDEN` before any hook is
+     * reached, so there is nothing for a hook to forward: see
+     * `content/docs/kernel/runtime-services/sharing-service.mdx`,
+     * "Enforcement is automatic — do not re-check it in a hook".
      * PRIVILEGE is judged by the security service on the ExecutionContext:
      * capability grants (`permissions`), placements (`positions`) and the
      * derived posture (ADR-0095 D3). A hook that re-decides access from this
@@ -443,10 +455,13 @@ export const HookContextSchema = lazySchema(() => z.object({
      */
     positions: z.array(z.string()).optional().describe(
       'Position names held by the caller (ADR-0090 D3; formerly `roles`), copied from '
-      + 'ExecutionContext.positions. For hook READS only — e.g. forwarding to the sharing '
-      + 'service as evaluation context. Authorization is decided by the security service on '
-      + 'the ExecutionContext (permissions / positions / derived posture); this is NOT an '
-      + 'authorization input and a hook must not gate a write by testing it.',
+      + 'ExecutionContext.positions. For hook READS only — e.g. tailoring a message, or '
+      + 'branching a business rule the hook runs through its own `ctx.api` channel. '
+      + 'Authorization is decided by the security service on the ExecutionContext '
+      + '(permissions / positions / derived posture); this is NOT an authorization input '
+      + 'and a hook must not gate a write by testing it. A hook context carries no '
+      + '`services` key, so the sharing service cannot be called from one either — the '
+      + 'sharing gates already ran inside the engine before the hook chain.',
     ),
     /**
      * Historical-import audit-preservation flag (#3493). Set by