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

pnpm-lock.yaml

@@ -4160,6 +4160,49 @@ importers:
         specifier: ^4.0.0-rc.7
         version: link:../../../vendor/cordis
 
+  packages/preset/agent-presets:
+    dependencies:
+      schemastery:
+        specifier: ^3.18.0
+        version: link:../../../vendor/schemastery
+    devDependencies:
+      '@cordisjs/plugin-include':
+        specifier: workspace:^
+        version: link:../../../vendor/include
+      '@cordisjs/plugin-loader':
+        specifier: workspace:^
+        version: link:../../../vendor/loader
+      '@deepseek-ai/dsh-agent':
+        specifier: workspace:^
+        version: link:../../core/agent
+      '@deepseek-ai/dsh-agent-loop':
+        specifier: workspace:^
+        version: link:../../core/agent-loop
+      '@deepseek-ai/dsh-invariants':
+        specifier: workspace:^
+        version: link:../../support/invariants
+      '@deepseek-ai/dsh-llm':
+        specifier: workspace:^
+        version: link:../../llm/llm
+      '@deepseek-ai/dsh-paths':
+        specifier: workspace:^
+        version: link:../../util/paths
+      '@deepseek-ai/dsh-scope':
+        specifier: workspace:^
+        version: link:../../core/scope
+      '@deepseek-ai/dsh-session':
+        specifier: workspace:^
+        version: link:../../core/session
+      '@deepseek-ai/dsh-system-prompt':
+        specifier: workspace:^
+        version: link:../../core/system-prompt
+      '@deepseek-ai/dsh-tools':
+        specifier: workspace:^
+        version: link:../../core/tools
+      cordis:
+        specifier: ^4.0.0-rc.7
+        version: link:../../../vendor/cordis
+
   packages/pty/pty:
     devDependencies:
       '@deepseek-ai/dsh-agent':