docs(automation): the flow authoring guide states the one node-id space (#16134)

`flows.mdx` said `id` was a "Unique node identifier" with no scope and
called a region "self-contained" with no id caveat — the two places an author
would now meet the parse refusal without warning. Both now say node ids are
one space across the top-level `nodes[]` and every region body, refused at
parse.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016N6xmWt5hYm94ffVEwGH8x

Author: claude

content/docs/automation/flows.mdx

@@ -134,7 +134,7 @@ Each node performs a specific action in the flow.
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| `id` | `string` | ✅ | Unique node identifier |
+| `id` | `string` | ✅ | Unique node identifier — unique across the **whole flow**: the top-level `nodes[]` and every region body (`loop.body`, `parallel.branches[]`, `try_catch.try` / `.catch`, at any depth) share one id space, and `FlowSchema` refuses a reused id at parse (`Duplicate node id …`, naming both locations) |
 | `type` | `string` | ✅ | Node type — a built-in id from the table above **or** a plugin-registered one. Per ADR-0018 the spec does not gate this with a closed enum; it is checked against the live action registry once that registry is complete — plugins contribute node types while they start, so flows registered during boot are checked in one pass when the vocabulary closes (all plugins started), and anything registered after that (Studio publish, dev reload) is checked immediately. Unknown types warn, never reject; executing one fails with `NO_EXECUTOR` |
 | `label` | `string` | ✅ | Display label |
 | `config` | `object` | optional | Type-specific configuration — the registered executor's `configSchema` owns its shape. Keys that schema does not declare are rejected at `registerFlow()`, and the built-in executors `parse()` the value against their Zod contract before running (#4277) |
@@ -496,7 +496,11 @@ a malformed construct is rejected before the flow can run.
 A region runs in the **enclosing variable scope** (the iterator value and any
 body mutations are visible to the surrounding flow) — it is *not* a separate
 `subflow` invocation. The container node's ordinary out-edges are the
-"after-loop / after-block" continuation.
+"after-loop / after-block" continuation. "Self-contained" describes control
+flow and variable scope, not ids: node ids are **one space across the whole
+flow**, so a region node may not reuse an id declared at the top level or in
+any other region — the collision is refused at parse (see `id` under
+[Node Structure](#node-structure)).
 
 ### Loop container