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>

test/path-picker-new-folder.test.ts

@@ -176,6 +176,7 @@ describe('PathPicker New folder', () => {
   it('is on for Link Existing and off for the other session-ui pickers', () => {
     const block = (name: string) => {
       const start = sessionUiJs.indexOf(`${name}() {`);
+      expect(start, name).toBeGreaterThan(-1);
       return sessionUiJs.slice(start, sessionUiJs.indexOf('PathPicker.open', start) + 400);
     };
     expect(block('openLinkCasePathPicker')).toContain('allowNewFolder: true');

test/routes/filesystem-mkdir-routes.test.ts

@@ -51,7 +51,8 @@ describe('POST /api/filesystem/mkdir', () => {
     try {
       const res = await mkdir({ path: casesDir, name: 'myproj' });
       expect(res.statusCode).toBe(400);
-      expect(res.json().error).toContain('Add Case > New Case');
+      expect(res.json().error).toContain('Pick a folder outside it');
+      expect(res.json().error).toContain('Add Case > Create New (no custom folder)');
       expect(existsSync(join(casesDir, 'myproj'))).toBe(false);
       // A folder inside a case is fine.
       mkdirSync(join(casesDir, 'real-case'));