chore(adr-anchors): anchor the field-readers module to ADR-0106 D7

The metadata-plane half of the ADR-0106 D7 paragraph, the configured
fallback set a caller with no set resolves, is the fallbackOnEmptySets
branch of resolveProjectionFieldMask, whose body moved to
security-plugin-field-readers.ts. The new entry carries ADR-0106 and that
paragraph of the security-plugin.ts entry verbatim, as the cbp and wall
module entries carry theirs. The security-plugin.ts entry is unchanged:
getReadableFields and getMetadataReadableFields stay there.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017BoLT8Gaa8t9a7AWVH8qZa

Author: claude

scripts/adr-anchors/packages__plugins__plugin-security__src__security-plugin-field-readers.ts.json

@@ -0,0 +1,7 @@
+{
+  "file": "packages/plugins/plugin-security/src/security-plugin-field-readers.ts",
+  "adrs": [
+    "ADR-0106"
+  ],
+  "invariant": "ADR-0106 D7 — getMetadataReadableFields differs from getReadableFields in exactly one place, and the asymmetry is the decision. On the DATA plane a caller resolving to zero permission sets falls OPEN, mirroring the engine middleware, because reporting a narrowing the data path would not enforce is its own drift. On the METADATA plane the same caller resolves the configured fallback permission set (the two-step /auth/me/permissions performs), so a guest-facing deployment's schema exposure is a deliberate permission-set decision rather than an accidental everything-default. Converging the two methods in either direction reverses this."
+}