#3115 Docs: serve authorization page should cover expression-reference checks and the vault.get trust boundary

The manual's swamp-serve authorization reference (content/manual/reference/swamp-serve/authorization.md) does not describe how serve authorizes CEL expressions, and its only vault mention points at #2676.

What changed and needs documenting:

• Since #2878 (release 20261006.193950.0), serve checks expression text against the principal who supplies it, when they supply it: model.create, model.edit and workflow.edit check every new or moved data.* / model. reference against the writer's data and model read grants, and model method run inputs get the same data rule. env in run inputs needs write on the model. Runs, evaluate and validate of stored content are never re-checked, so running a workflow an admin wrote needs no extra grants.
• Per #3086, a vault.get(...) expression is an author's capability and is not checked on any serve path: writers may use any vault secret, and model method run inputs may carry vault.get from any caller (masked in output). So write on workflows or models, and run on a model whose method can surface its inputs, include access to the repo's vault secrets. This is distinct from the vault.get serve request, which needs read on data:vault. Workflow run inputs are inert.

Suggested content: a section on the authorization page (e.g. Expressions and grants) with the table of what is checked when, the vault/env author-trust boundary, and guidance to grant write on workflows/models only to principals trusted with the repo's secrets. Link to the existing cel-expressions note on model method run inputs being evaluated.

Source of truth: design/enablers/access-control.md, section Expression references, in swamp-club/swamp.