chore(spec): record STACK_KEY_GUIDANCE in the API-surface snapshot (#4167)

`check:api-surface` failed on the #4167 commit: exporting `STACK_KEY_GUIDANCE`
adds to `@objectstack/spec`'s public surface, and the generated snapshot is the
gate's baseline.

    ./data
      + STACK_KEY_GUIDANCE (const)
    @objectstack/spec public API changed: 0 breaking (removed/narrowed), 1 added.

Regenerated with the command the gate names. The diff is one line — the export
is additive, alongside the FIELD_KEY_GUIDANCE / OBJECT_KEY_GUIDANCE entries it
sits with, and nothing was removed or narrowed.

Also ran the rest of the gates CI runs, so this does not take another round:
doc-authoring, role-word, org-identifier, authz-resolver, route-envelope,
error-code-casing, wildcard-fallthrough, release-notes, node-version, plus the
spec package's authorable-surface, spec-changes, upgrade-guide, liveness and
docs checks — all green.

Refs #4167

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HrRNgrWaRtggzmrHpbomyh

Author: claude

packages/spec/api-surface.json

@@ -499,6 +499,7 @@
     "SQLiteDataTypeMappingDefaults (const)",
     "SSLConfig (type)",
     "SSLConfigSchema (const)",
+    "STACK_KEY_GUIDANCE (const)",
     "STRING_VALUE_TYPES (const)",
     "STRUCTURED_JSON_TYPES (const)",
     "Scalar (type)",