docs(changeset): cover the empty FLS remainder

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012GKcPZbMoGq7WPzKLfRBTU

Author: claude

.changeset/security-fls-unknown-field.md

@@ -10,6 +10,7 @@ The failure direction is **fail open**, and this spelling is the one that accumu
 
 - **A second rule, not a widening of the first.** `security-fls-unqualified-key` is correct inside its declared scope and is untouched; the two defects have different prescriptions (add the object prefix / fix the field name) and suppressing one must not suppress the other. Two ids, two messages.
 - **Where the existence answer comes from.** The rule resolves through `object-graph.ts`, the shared index every field-existence rule in this package already uses — no new input path. It therefore inherits that module's three skips, each of which is the difference between a finding and a false one: an object this stack does not define (it may be another installed package's), an object with no readable field map (an ADR-0015 `external` object, an introspected datasource), and registry-injected system columns such as `created_at` or `owner_id`, which are real at runtime and appear in no authored `fields`.
+- **A truncated key is the same defect and is reported by the same rule.** `fields: { 'crm_account.': … }` passes the runtime's prefix test and resolves to the empty column name, so it matches nothing exactly as a dangling name does. `PermissionSetSchema.fields` is `z.record(z.string(), FieldPermissionSchema)` — a bare string key with no pattern and no refinement — and this rule is the only reader of those keys, so before this change nothing reported it at all. A key naming an object this stack does not declare still falls to skip 1, truncated or not.
 - **It mirrors the evaluator, including on a multi-dot key.** Only the first dot separates object from field, because `ObjectSchema.name` is `/^[a-z_][a-z0-9_]*$/` and cannot contain one. `'crm_account.owner.name'` therefore asks for a column literally named `owner.name` and is reported: FLS keys address columns, never joins, and resolving that as a relationship hop would have been a fail-open divergence from the gate the rule mirrors.
 
 **What moves for consumers.** A stack carrying a dangling FLS key built clean before and now fails `os validate` / `os compile`, and is refused at the runtime publish door for `permission` and `object` writes (this rule joins the existing `validateSecurityPosture` registration; no new registry entry). That is the point — the key was never enforcing anything. A stack whose FLS keys all resolve is byte-identically clean: measured on the shipped showcase, whose six authored keys emit zero findings, with a firing control (one injected dangling key produces exactly one finding) beside the zero.