docs(spec): mark ToolSchema as a read-only projection, not an execution entry point (#1892)

#1892 tool disposition (ADR-0049 line): the ledger already documents that
tool metadata is a one-way, write-only projection (no executor loads a
metadata-authored tool; the runtime uses a separate AIToolDefinition in
cloud service-ai). Surface that on the SPEC itself so an author/AI reading
the Zod schema — not just the ledger — knows a hand-authored tool will not
run in the open edition. Non-breaking describe-only change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LddW4NaQBdf5FTEnBPpnUJ

Author: claude

content/docs/references/ai/tool.mdx

@@ -27,6 +27,8 @@ const result = Tool.parse(data);
 
 ## Tool
 
+AI tool definition. [READ-ONLY PROJECTION — not an execution entry point] Authoring a tool as metadata does NOT make it runnable: this schema has no `implementation`/`handler` field and no framework executor loads a metadata-authored tool. The runtime executes a separately-registered `AIToolDefinition` (cloud `@objectstack/service-ai`); tool metadata is a one-way projection for Studio/discovery. Do not expect a hand-authored tool to run in the open edition (liveness audit #1878/#1892).
+
 ### Properties
 
 | Property | Type | Required | Description |

packages/spec/src/ai/tool.zod.ts

@@ -117,7 +117,7 @@ export const ToolSchema = lazySchema(() => z.object({
   // ADR-0010 — runtime protection envelope (internal — set by loader).
   ...MetadataProtectionFields,
 
-}));
+}).describe('AI tool definition. [READ-ONLY PROJECTION — not an execution entry point] Authoring a tool as metadata does NOT make it runnable: this schema has no `implementation`/`handler` field and no framework executor loads a metadata-authored tool. The runtime executes a separately-registered `AIToolDefinition` (cloud `@objectstack/service-ai`); tool metadata is a one-way projection for Studio/discovery. Do not expect a hand-authored tool to run in the open edition (liveness audit #1878/#1892).'));
 
 export type Tool = z.infer<typeof ToolSchema>;