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.

.gitignore

@@ -107,7 +107,9 @@ todo.md
 @fix_plan.md
 readme-preview.mjs
 
-# Uploaded images land here under each session working dir (runtime artifact)
+# Prompt uploads land here under each session working dir (runtime artifact);
+# .claude-images/ is where they landed before the move.
+.codeman-uploads/
 .claude-images/
 
 # Local-LLM harness smoke-test config (real IPs/keys) — see the .example.json