docs: describe deletes that go to the trash

A new page, `doc/trash.md`, covers what a delete does now: where a path lands on
each platform, what is still deleted outright (an explicit `permanent=true`, a
host that cleared `[trash]`, scratch paths under the OS temp dir, the
`copy_path` / `move_path` overwrite, the temporary-workspace cleanup), what
happens when a trashing is refused — the file stays and the error says how to
delete it on purpose — and the two things the trash cannot help with: a shell
`rm` and anything a plugin or MCP server does on its own.

`[trash]` joins the example config and the README snippet, the README gains the
one-paragraph version, `AGENTS.md` the tool/config notes (including the confirm
wording a delete now uses), and the default tool playbook tells the model that
`delete_path` is undoable and what `permanent=true` costs.

doc/architecture.md

@@ -45,4 +45,6 @@ Images (a turn's pictures, a tool result's pictures, what each wire surface
 takes): [images.md](./images.md).
 Prompt notifications (a prompt that waits on the human, and how it can be
 answered from the notification): [notifications.md](./notifications.md).
+Deletes (a `delete_path` call moves the path into the system trash, and what
+that does not cover): [trash.md](./trash.md).