`dsh-system-prompt` owns the deployment persona as its own config and registers
that section unconditionally, so a process has exactly one. An agent preset
cannot mount the prompt registry itself, which means that without a row of its
own a preset could change an agent's tools but never its identity — and a
roster of presets that all sound the same is not worth having.
The row is scope-only by construction: mounted outside an agent scope it
collides with the registry's own `deployment:persona` registration and fails
loud. That is the correct shape rather than a gap, because the unscoped slot
already has an owner and this row exists to shadow it for one agent.
Two behaviours are pinned by test because both read the other way at a glance:
an empty persona still occupies the slot (shadowing the deployment persona away
entirely, then disappearing at render), and `assemble()` keeps section text
uninterpolated — `renderPrompt()` is the stage that resolves `{{…}}`.
A preset is a directory holding one `agent.cordis.yml`. Mounting it under an
agent's scope context during `setup(agentCtx)` gives that one session its own
tools and prompt sections while every other live session keeps its own.
No registry gains a tier. `dsh-tools` and `dsh-system-prompt` already file
registrations into the calling context's scope layer, and entry contexts chain
to the context a subtree was plugged into, so a composition mounted under
`agent.ctx` is that agent's alone and unwinds with it.
The mount audits itself because a directly-plugged subtree is absent from
`ctx.loader.entries()` and no boot audit covers it. It rejects an unscoped
target, a row that never became usable, and a row that published a service into
the root service realm — that last one is process-global rather than
per-session, and its collision with the next session surfaces as an unhandled
rejection `setup` never observes, leaving a half-composed agent that looks
healthy. The package invariant re-checks that rule on every service
notification, since a row publishing from a timer would escape a one-shot audit.
Raises the `packages/README.md` word ceiling from 920 to 980: the group table
must enumerate every group, and the new `preset/` row is necessary content.
Design: .agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md
