feat(agent-presets): compose each session's agent from a preset cordis.yml

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

tsconfig.base.json

@@ -86,6 +86,7 @@
         "./packages/goal/*/src/invariant.ts",
         "./packages/guard/*/src/invariant.ts",
         "./packages/plan/*/src/invariant.ts",
+        "./packages/preset/*/src/invariant.ts",
         "./packages/subagent/*/src/invariant.ts",
         "./packages/tasks/*/src/invariant.ts",
         "./packages/workflow/*/src/invariant.ts",
@@ -185,6 +186,7 @@
         "./packages/goal/*/src",
         "./packages/guard/*/src",
         "./packages/plan/*/src",
+        "./packages/preset/*/src",
         "./packages/subagent/*/src",
         "./packages/tasks/*/src",
         "./packages/workflow/*/src",