feat(cli): build the cross-producer sys_secret reference union (#12663)

.changeset/sys-secret-cross-producer-reference-union.md

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+---
+"@objectstack/cli": patch
+---
+
+feat(cli): build the cross-producer `sys_secret` reference union — the primitive a safe orphan sweep needs (#12663)
+
+`sys_secret` has three privileged producers, each holding its handle in a
+column of its own: `SettingsService` (a bare `sec_…` in
+`sys_setting.value_enc`), the engine's `secret`-typed field channel
+(`secret:<id>` on an arbitrary business row) and the datasource credential
+binder (`sys_secret:<id>` at a datasource artefact's
+`external.credentialsRef`). Nothing enumerated all three, so the only sound
+deletion predicate — "attributable AND unreferenced by the COMPLETE union" —
+had no union to stand on. `packages/cli/src/utils/secret-reference-union.ts`
+is that union, read-only across all three surfaces.
+
+Why the shipped report-only classifier is not enough, reproduced against real
+code in the new test file: `classifySysSecretRows` attributes a row by
+`(namespace, key)` membership in the settings manifests' encrypted specifiers,
+and that is a **name match, not ownership** — `sys_secret` carries no producer
+column and the three producers write those two columns with three different
+meanings. A live, engine-owned credential on an object named like a settings
+namespace, with a field named like a specifier key, classifies `orphaned`
+today. There is no recovery from acting on that: the audit trail records
+digests, not handles, so nothing can name the destroyed handle afterwards.
+
+Completeness is therefore the whole contract, and it is structural rather than
+asserted:
+
+- the family set is closed and the assembler takes a `Record` over it, so
+ omitting a producer is a type error, not a smaller union;
+- a read that did not happen is a declared **gap**, never an empty answer — a
+ missing driver, a throwing read, an unparseable artefact, or a host that did
+ not declare its code-defined datasources all make the union
+ `complete: false`, and `assertSecretReferenceUnionComplete` refuses it with
+ the ADR-0112 pair `PRECONDITION_REQUIRED` / 428;
+- family 2 is enumerated from the metadata registry on every call, because its
+ holders are every `secret`-typed field on every registered object,
+ tenant-authored ones included — a newly registered secret field is in the
+ union with no code change.
+
+Reads go to the driver through the engine's public `getDriverForObject()`: the
+`secret:` ref only exists at that level (the read path masks it
+unconditionally), and any scoped read would silently under-report — the
+direction that deletes live credentials.
+
+`patch` rather than `minor`: this adds no surface to the package's entry
+barrel and no command or flag. It is an internal primitive whose named reader
+is the deletion half of #8103, in this same package; publishing it as an
+external API is a separate decision with its own changeset. Read-only by
+construction — nothing here writes, deletes or decrypts, and it contains no
+deletion command, dry-run or sweep.