fix(picker): word the cases-folder refusal for both pickers, and finish the review items

With the folder field empty, Create New's Browse opens in the cases folder, and New
folder there answered "Use Add Case > New Case". There is no tab of that name (the
label is Create New), and a Create New user is already on it. The refusal now reads
right from Link Existing and from Create New's custom folder. Its test pin, the code
comment, the manual, api-reference, CLAUDE.md and architecture-invariants move with it.

The block() helper in the New folder picker test returned an empty string for a name
that is not in session-ui.js, so .not.toContain passed and a renamed picker dropped out
of the check. It now asserts the name was found.

The manual's path picker list said two places and left out Create New, which the New
folder paragraph names. It now lists every Browse that opens the picker, and that
paragraph is rewrapped to the page's width.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/api-reference.md

@@ -905,10 +905,10 @@ html and svg are the ones deliberately kept out.
 `POST /api/filesystem/mkdir` with `{ path, name, sessionId?, showHidden? }` makes one
 folder directly inside `path`, which must be a folder the path picker may browse, and
 answers `{ path }`. `name` is one path segment (letters, numbers, spaces and `_ - . ~`, not
-starting with a dot or `~`). `400` for a cases folder (use Add Case → New Case) or a `path`
-that is not a directory, `403` for a blocked tree or a `path` outside the browse roots,
-`404` for a `path` that does not exist, `409` when the folder already exists, and `422`
-when the mount under `path` does not answer.
+starting with a dot or `~`). `400` for a cases folder (use Add Case → Create New with no
+custom folder) or a `path` that is not a directory, `403` for a blocked tree or a `path`
+outside the browse roots, `404` for a `path` that does not exist, `409` when the folder
+already exists, and `422` when the mount under `path` does not answer.
 
 ## Rendered HTML files