fix(picker): do not offer New folder inside a cases directory

A folder made in a cases directory is already a case (no scaffold), which Link
Existing then refuses by name and which deleting would take the linked folder
with. The route refuses a cases directory as the parent (the shared one and the
caller's own), a leading tilde in the name, and maps a vanished parent to 404.
Docs now say every name it accepts is one Link Existing accepts, and the wiki
describes the button.

docs/architecture-invariants.md

@@ -429,7 +429,7 @@ The terminal provider scans `terminalLinkLine()`, which extends `terminalPathLin
 
 ### Filesystem path picker
 
-**Filesystem path picker** (Link Existing "Browse" button + the extended mobile keyboard's `📁 Path` key): a lazy one-directory-at-a-time browser over `GET /api/filesystem/browse`, with `GET /api/filesystem/preview` serving the tapped file. It starts at the active session's working directory (falling back to the Codeman Cases root, then `/mnt/d`, then the first root), hides dot entries, and inserts the chosen path **without** Enter so the prompt is not submitted. The companion `⌫ All` key clears only the current unsent prompt buffer and must never emit the agent's `/clear` command. **New folder** (opt-in per caller, `allowNewFolder`; Link Existing has it): `POST /api/filesystem/mkdir` `{ path, name, sessionId? }` creates ONE folder (non-recursive) directly inside a folder the picker may browse, resolved through the same roots, ownership and blocked-tree rules as browse (`403` outside them or in a blocked tree or unwritable, `404` missing parent, `409` exists, `400` a parent that is a file, or a name outside letters, digits, space and `_ - . ~`, containing `..`, starting with a dot or over 255 bytes, so every folder it makes is one Link Existing accepts; `showHidden` is forwarded like browse's); the picker reloads the folder and selects the new one. A picker that browses somewhere this route cannot create in (the container-workdir picker, `fetchListing`) must not set the flag.
+**Filesystem path picker** (Link Existing "Browse" button + the extended mobile keyboard's `📁 Path` key): a lazy one-directory-at-a-time browser over `GET /api/filesystem/browse`, with `GET /api/filesystem/preview` serving the tapped file. It starts at the active session's working directory (falling back to the Codeman Cases root, then `/mnt/d`, then the first root), hides dot entries, and inserts the chosen path **without** Enter so the prompt is not submitted. The companion `⌫ All` key clears only the current unsent prompt buffer and must never emit the agent's `/clear` command. **New folder** (opt-in per caller, `allowNewFolder`; Link Existing has it): `POST /api/filesystem/mkdir` `{ path, name, sessionId? }` creates ONE folder (non-recursive) directly inside a folder the picker may browse, resolved through the same roots, ownership and blocked-tree rules as browse (`403` outside them or in a blocked tree or unwritable, `404` missing parent, `409` exists, `400` a parent that is a file, or a name outside letters, digits, space and `_ - . ~`, containing `..`, starting with a dot or a tilde or over 255 bytes, so every name it accepts is one Link Existing accepts, and `400` a parent that is a cases directory, where a folder is already a case (Add Case > New Case makes one there); `showHidden` is forwarded like browse's); the picker reloads the folder and selects the new one. A picker that browses somewhere this route cannot create in (the container-workdir picker, `fetchListing`) must not set the flag.
 
 ⚠️ **This is a second file-serving surface, so it carries the same confinement burden as [Attachments](#attachments) and does not inherit it automatically.** Traversal is allowlisted to Home, `CASES_DIR`, `/mnt/d`, or extra roots explicitly configured via `CODEMAN_FILE_PICKER_ROOTS`; sensitive trees are blocked and symlink escapes are rejected after `realpath` resolution rather than before. Without the realpath step a symlink inside an allowed root would walk straight out of it. `preview` reuses the shared conversion cache and the **global** `document-conversion-limiter`, which is what stops N concurrent large-document previews from forking N multi-minute converter processes. Content types are pinned: images and PDF inline, DOCX/PPTX through the converters, and Markdown/TXT/JSON as inert `text/plain` (never `text/html`, which would be stored XSS on our own origin). Size caps are 2MB for text and 50MB for binary/document previews.
 

docs/wiki/Working-With-Files.md

@@ -159,6 +159,8 @@ For choosing a path rather than typing one. It appears in two places:
 - **Browse** in **Add Case → Link Existing**.
 - The **📁 Path** key on the mobile keyboard bar.
 
+In **Link Existing**'s picker, **+📁** (New folder) makes a folder inside the folder you are viewing and selects it. Names use letters, numbers, spaces and `_ - . ~` (not starting with a dot or `~`), and it is not offered inside the cases folder, where every folder is already a case.
+
 It browses one directory at a time and can show hidden entries on request. The current
 folder is an editable field: type or paste a path and press Enter (or **Go**) to jump
 straight there, and a full file path lands in its folder with that file selected. The