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

docs/capability-seams.md

@@ -80,6 +80,8 @@ flowchart LR
   svc_userInteraction["ctx.userInteraction<br/>Human question/answer seam"]
   pkg_plan_mode["plan-mode"]
   svc_planMode["ctx.planMode<br/>Plan collaboration state"]
+  pkg_agent_presets["agent-presets"]
+  svc_agentPresets["ctx.agentPresets<br/>Per-session agent composition"]
   pkg_commands["commands"]
   svc_commands["ctx.commands<br/>Human command registry"]
   pkg_session_projection["session-projection"]
@@ -168,6 +170,7 @@ flowchart LR
   pkg_acp --> svc_approval
   pkg_agent --> svc_agents
   pkg_agent_loop --> svc_agentLoop
+  pkg_agent_presets --> svc_agentPresets
   pkg_approval --> svc_approval
   pkg_bash --> svc_bash
   pkg_bash_env --> svc_bashEnv
@@ -370,6 +373,7 @@ flowchart LR
 | `ctx.tools` | `core` | [`tools`](../packages/core/tools) | - | [`agent-loop`](../packages/core/agent-loop), [`tool-ask-user`](../packages/ui/tool-ask-user), [`tool-bash`](../packages/bash/tool-bash), [`tool-cordis`](../packages/cordis/tool-cordis), [`tool-fs`](../packages/fs/tool-fs), [`tool-pty`](../packages/pty/tool-pty), [`tool-skill`](../packages/skill/tool-skill), [`tool-subagent`](../packages/subagent/tool-subagent), [`tool-todo`](../packages/todo/tool-todo), [`tool-web`](../packages/web/tool-web) | - | Registers capabilities, owns Code Mode transport, and routes calls through pre-policy, monotonic guards, around dispatch, post-policy, and final-result observation. |
 | `ctx.userInteraction` | `seam` | [`user-interaction`](../packages/ui/user-interaction) | - | [`tool-ask-user`](../packages/ui/tool-ask-user) | - | UI front doors provide the active human-answer provider; tool-ask-user pauses a tool call on the provider-neutral ask() promise. |
 | `ctx.planMode` | `core` | [`plan-mode`](../packages/plan/plan-mode) | - | - | - | Folds logged plan/mode state, flushes user selections at turn boundaries, renders deployment-owned guidance, registers /plan, and keeps the plan-exit schema stable across transitions. |
+| `ctx.agentPresets` | `core` | [`agent-presets`](../packages/preset/agent-presets) | - | - | - | Discovers profile directories over trusted and user-authored roots and mounts one profile cordis.yml under an agent scope during creation, rejecting a row that never activates or that publishes into the root service realm. |
 | `ctx.commands` | `core` | [`commands`](../packages/ui/commands) | - | - | - | Plugins register direct human commands without sending invocations to the model. |
 | `ctx.sessionProjections` | `core` | [`session-projection`](../packages/session-projection/session-projection) | - | [`tool-todo`](../packages/todo/tool-todo), [`session-title`](../packages/session-title/session-title), [`host-apiproxy`](../packages/host/apiproxy) | - | Domains register state-driven fold units; the eager drive keeps per-session watermark states and api-proxy serves baselines and pushes changed values. |
 | `ctx.sessionProjectionCache` | `core` | [`session-projection-cache`](../packages/session-projection/session-projection-cache) | - | [`host-apiproxy`](../packages/host/apiproxy) | - | Durably checkpoints projection unit states per session (throttled + turn/end/detach mandatory points) and serves the cold-read ladder: cache row + persistence tail replay, so listings never load full logs. |