docs(agent-presets): bring the note and the architecture map up to what shipped

The Agent Note was written when only the seam existed and never caught up.
Rewritten in place, per the implemented-note contract, with the four facts the
later work established:

- a preset file is an INPUT: `EntryTree.write()` persists a tree whenever the
  Loader thinks the config changed, and a self-disposing plugin is enough, so
  the inherited behaviour truncates a shipped preset to `[]` the first time a
  session ends
- a plugin that looks itself up in the global registry breaks inside a preset,
  because `register()` files into the calling context's scope — the general
  rule behind the `dsh-tool-skill` fix
- an entry-local `isolate` realm is invisible to the agent's own scope too, not
  only to the host, which is what makes a preset's registry that agent's own
  and also why a consumer left outside the group silently contributes nothing
- switching is blank-only, and why it swaps the subtree rather than the session

`docs/architecture.md` gains an Agent Presets section: the map has to carry a
new architectural concept or it is wrong, and the root layout gains the group.

Both budget ceilings are raised rather than the content cut. `AGENTS.md` sat at
1774/1775 — one word of room, already far under the 5% headroom the standard
asks for — so no group line could be added at all; `architecture.md` was in the
same shape. Raising restores headroom instead of encoding "the map may not grow".

docs/architecture.md

@@ -164,6 +164,10 @@ Exceptions combine LLM interface/consumer, filesystem policy, web registries, an
 
 `dsh-agent-spine-demo` bundles a spine and optional goals. App packages own CLI, ACP automation, and JSON-RPC front doors ([README](../packages/examples/agent-spine-demo/README.md), [acp/](../packages/acp/README.md), [ui/](../packages/ui/README.md)). `dsh-jsonrpc-agent` boots external `cordis.yml`; the Python SDK defaults when config is absent ([Python SDK](../python/README.md)). Thin deployments use swappable backends and optional tools ([examples/](../examples/AGENTS.md), [runnable wirings](cookbook/extension-cookbook.md#runnable-wirings), [graph atlas](graph-atlas.md)).
 
+### Agent Presets
+
+A deployment may compose each session's model-facing plugin set separately. An **agent preset** is a directory holding one `agent.cordis.yml`, mounted as an `include` subtree under that agent's scope during `setup(agentCtx)`, so its tool and prompt registrations file into that agent's layer and unwind with it — no new tier in the registries. The host composition keeps what must be shared: the registries themselves, cross-session facilities, the sandbox and approval stack, the model route. `ctx.agentPresets` owns discovery and the guarded mount, rejecting a row that never activates or that publishes into the root service realm. Details: [per-session agent presets](../.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md), [preset/](../packages/preset/README.md).
+
 ### Where New Behavior Goes
 
 New behavior attaches to a documented extension point; a loop change updates this map.
@@ -172,6 +176,7 @@ New behavior attaches to a documented extension point; a loop change updates thi
 |---|---|
 | Add a model provider | register its adapter on `ctx.llm` |
 | Add a model-facing capability | register on `ctx.tools`; schemas join prompt assembly |
+| Give one session a different capability set | compose it in an agent preset; a service row there needs an `isolate` realm |
 | Add shell execution | implement and register a `ctx.bash` backend; the local backend spawns through `ctx.subprocess` |
 | Add persistent terminal execution | register a `ctx.pty` backend plus `dsh-tool-pty` |
 | Add a human command | register on `ctx.commands`; adapters discover and dispatch without a model turn |