spec(docs): mark `PromptTemplate.system` / `.user` as `[EXPERIMENTAL — not enforced]` and enrol the marking in the D7 ledger (#15954 ruling, option B for the template family)

[os-zhuang]

Work card for the #15954 ruling (director seat, decision batch #56, 2026-09-06): the template-typed pair is marked, not retired.

What to do

• packages/spec/src/ai/model-registry.zod.ts:121-122 — prefix both .describe() strings with the same [EXPERIMENTAL — not enforced] form ai/knowledge-source.zod.ts uses for KnowledgeRefreshPolicy.cron, stating that no runtime renders or executes the template today.
• D7 ledger (packages/qa/dogfood/test/expression-conformance.ledger.ts): the two template-prompt rows record the marking (the row shape the knowledge-refresh row already uses).
• Regenerate the reference pages that carry the describe text.
• ⚠️ PromptTemplate.user is required (no .optional()). Do not change that here — a retirement or optionalisation of a required key is a parse-breaking change and would be its own card.
• Changeset: patch on @objectstack/spec (describe text on a published package). Clause-②: no (prose only), cite this ruling.

Re-check before claiming

grep -n "TemplateExpressionInputSchema" packages/spec/src/ai/model-registry.zod.ts
grep -n "EXPERIMENTAL" packages/spec/src/ai/knowledge-source.zod.ts