feat(cases): create a new case in a custom folder

POST /api/cases takes an optional path; Add Case > Create New gets a 'Create in a
custom folder' option with Browse. The folder is created (or an empty one filled),
scaffolded like a normal case and registered as a linked case. System, home,
credential and Codeman folders are refused; a folder with files is Link Existing's
job; a failure after the first write undoes what this call created. Admin only in
multi-user mode, like Link Existing.

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

docs/api-reference.md

@@ -700,6 +700,20 @@ normal `caseName`/`mode`/etc. body)
   jarring than a full relaunch, and folding it into the one-shot path is
   separate work — see `docs/custom-model-endpoints-plan.md`).
 
+## Creating a case in a custom folder
+
+`POST /api/cases` takes `{ name, description?, path? }`. Without `path` it creates `<cases dir>/<name>` as always. With `path` (absolute, or starting with `~`) the case folder is created at that exact path instead, scaffolded the same way (`CLAUDE.md`, `src/`, `.claude/settings.local.json`), and registered in the linked-cases registry, so it lists, resolves and deletes like a linked case (deleting unlinks; it never removes files). Response: `{ case: { name, path } }`, where `path` is the symlink-resolved folder.
+
+The target is judged before anything is written:
+
+- It must be absolute with no `..` and none of the shell metacharacters a session working directory is rejected for (spaces are fine). `400 INVALID_INPUT` otherwise.
+- It must not be a system directory (`/etc`, `/usr`, `/proc`, ...), the home folder itself, Codeman's own data folder, or a credential/config tree (`~/.ssh`, `~/.aws`, `~/.claude`, ...). Judged on the path as typed and on its symlink-resolved form. `400`.
+- Its parent must already exist (one folder is created, never a chain): `404 NOT_FOUND`.
+- The folder must not exist, or must be an **empty** directory; a folder with contents is Link Existing's job: `409 ALREADY_EXISTS`. A symlink or a plain file at the target is `400`.
+- `409 ALREADY_EXISTS` also for a case name already in use (in the cases dir or the registry) and for a folder that is already a case.
+
+Admin only in multi-user mode (`403`), like `POST /api/cases/link`: it writes outside the cases directory and into the shared, ownerless registry. If anything fails after the first write, what this call created is removed (the whole folder if it created it, otherwise only the scaffold inside the empty folder you picked) and the response is `500`.
+
 ## CLI management
 
 Read and write the CLI registry (`docs/cli-registry.md`). Every **write** route answers `403 FORBIDDEN` while `cliManagementEnabled` is off (the default), and for a non-admin in multi-user mode. A write that would overwrite a `clis.json` which does not parse, or which has group/world permission bits, is refused with `409 CONFLICT` and a message naming the fix; the file is left untouched.

docs/wiki/Core-Concepts.md

@@ -19,7 +19,7 @@ Three ways to get one, all under **+** next to the case picker:
 
 | How               | Result                                                                                                 |
 | ----------------- | ------------------------------------------------------------------------------------------------------ |
-| **Create New**    | A fresh `~/codeman-cases/<name>` with a scaffolded `CLAUDE.md`.                                          |
+| **Create New**    | A fresh `~/codeman-cases/<name>` with a scaffolded `CLAUDE.md`, or with **Create in a custom folder**, a new folder inside a parent you choose, scaffolded the same way and registered in place like a linked case. |
 | **Clone Repo**    | A repo cloned into `~/codeman-cases/<name>` and registered as a case. Private repos need this machine's own git credentials (see below). |
 | **Link Existing** | An existing folder anywhere on disk, registered in place. Nothing is copied or moved.                    |
 

docs/wiki/Quick-Start.md

@@ -42,7 +42,7 @@ To make a new one, click **+** next to the picker. The Add Case dialog has three
 
 | Tab               | Use it when                                                                                                        |
 | ----------------- | ------------------------------------------------------------------------------------------------------------------ |
-| **Create New**    | Starting a fresh project. Creates `~/codeman-cases/<name>` and scaffolds a `CLAUDE.md` into it.                     |
+| **Create New**    | Starting a fresh project. Creates `~/codeman-cases/<name>` and scaffolds a `CLAUDE.md` into it. Tick **Create in a custom folder** to put it somewhere else instead. |
 | **Clone Repo**    | Working on an existing repo: public, or private once this machine's git can authenticate (the Docker image can include `gh`/`az` helpers for this). Paste the URL; Codeman preflights it as you type, offers the repo's real branches and tags, and fills in the case name. |
 | **Link Existing** | The code is already on disk. Point at the folder, with **Browse** if you would rather click than type.               |