The regression for `PresetTree.write` lived two layers up, so this layer's own
assertion could not fail: it checked the file after an ordinary teardown, and
the Loader's unload listener returns early when the whole tree is being
disposed, so the override never ran. Moves the self-disposing-row test down to
the layer that adds the override, in a temp root so a committed fixture cannot
be damaged by the run that proves the bug.
master made `allowParallelInProgress` a required field on dsh-tool-todo, so
every composition must choose it. The base patch was updated with the field,
but the Web surface takes `tool-todo` from the mounted preset instead, and
that row carried no config — so the `standard` preset failed to mount and
every session on the Web surface died at setup.
Also make the preset tree's write test reach the override. Tearing the agent
down stops in the loader's own "tree is being disposed" case before any write;
a live row reconfiguring itself is the trigger that actually gets there.
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
