docs(skills): use policies as the phantom-key example in the quoted refusal

The quoted defineStack refusal used the ADR-0090 D3 reserved word as its
example key, which added a third occurrence to a file baselined at two and
red-lit check:role-word. The sibling example in the sentence above it is
already `policies:`, so the quote now uses that instead.

Not an edited quote: the refusal was re-measured live against the built
@objectstack/spec with `policies` as the offending key, and the output is
reproduced as measured. The two pre-existing occurrences are untouched, and
the baseline is not modified — the ratchet returns to 2 by removing the new
occurrence, which is the author-side remedy.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeA3nU1B5Q2pgxqxgUrexd

Author: claude

skills/objectstack-platform/SKILL.md

@@ -192,7 +192,7 @@ no-op — the top level refuses it and the stack fails to load:
 ```
 defineStack validation failed (1 issue):
 
-  ✗ (root): Unrecognized key(s) on this stack definition: `roles`. …
+  ✗ (root): Unrecognized key(s) on this stack definition: `policies`. …
 ```
 
 Undeclared keys are handled per surface, and the two postures are worth