feat(uploads): write prompt uploads to a hidden, self-ignoring .codeman-uploads/ folder

A pasted image landed in <workspace>/.claude-images/, a name that belongs to another tool, in a folder nothing ignored, so it showed up in git status of every case that ever received a paste. Uploads now go to a flat <workspace>/.codeman-uploads/ that carries a .gitignore of `*` (written once, never over a file already there): in the workspace because that is the only path identical for a local agent and a container, flat because a nested .codeman/ is the data dir itself when the workspace is the home directory.

The directory names live in paste-image-gc.ts alone. The old folder receives nothing but stays readable for one release: the hourly sweep and the delete cleanup go through uploadDirs(), the image watcher's ignore filter reads the names. uploadDirs() lists only real directories, none that is, contains or sits inside the data dir, and nothing for a remote session, since both consumers delete. The route refuses a remote (SSH) session before touching disk: the file would land on this host under the remote path, where the agent cannot read it.

test/paste-image-dir-shared.test.ts binds port 0 and leaves the port guard's legacy list.

Decided in #553.

docs/api-reference.md

@@ -466,6 +466,21 @@ geometry was read. The capture runs synchronous tmux calls on the server; the
 | `tail=<bytes>` | Keep the newest `<bytes>` of the result (`truncationReason: 'tail'` when it cut). |
 | `lines=<n>` | With `full=1` only: read at most `<n>` lines of tmux history above the visible frame. An integer of at least 1, clamped to the configured history limit; absent or malformed, the whole limit (100,000 lines by default), as before. `truncated` and `truncationReason` describe byte cuts only, not this bound. Without it a full capture reads all of that history before `tail` cuts it, so a client that keeps a fixed number of lines (the tile grid sends its xterm's scrollback plus its rows) should send it. |
 
+## Prompt uploads (`POST /api/v1/sessions/:id/paste-image`)
+
+A `multipart/form-data` body with one `image` part. The file is written into the
+session's workspace as `<workingDir>/.codeman-uploads/paste-<ms>-<hex>.<ext>`, and
+`data` carries `path` and `filename` for the client to type the path into the
+prompt. The folder is Codeman's own: hidden, created on first use with a
+`.gitignore` containing `*` (written once, never over a file already there), and
+cleaned up the way pasted images always were: `paste-*` files older than 7 days
+go in an hourly sweep, and the folder goes when the last session of that
+workspace is killed. Uploads made before this release sit in `.claude-images/`;
+that folder receives nothing new, and is swept and removed the same way for one
+release. A remote (SSH) session answers 400, since the file would land on the
+Codeman host under a path the remote agent cannot read; Docker sessions are
+fine, their workspace is bind-mounted at the same absolute path.
+
 ## Session lineage (`parentSessionId`)
 
 A create request may name the session that spawned it, which the web UI draws as a