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".

AGENTS.md

@@ -28,6 +28,7 @@ packages/    @deepseek-ai/dsh-<pkg> workspaces at packages/<group>/<pkg>/
   workflow/    workflow seam + worker-thread engine + workflow tool
   todo/        todo_write tool
   plan/        plan mode as logged per-agent collaboration state
+  preset/      per-session agent composition from preset cordis.yml files
   guard/       loop-hygiene plugins
   cordis/      self-referential toolset: the agent inspects/mounts plugins in its own runtime
   hooks/       Claude Code/Codex hook bridges + shared wire-protocol library