fix(docker): verify the container workdir and end the probe with exit 0

Two defects that only a real container exposes.

The probe chained `command -v X && echo X` with semicolons, and a script's exit
status is its last command's. A container without the last probed CLI made the
whole `sh -lc` exit 1, so a perfectly healthy container with tmux and claude was
reported as "could not exec into the container". A missing CLI is data here, not
failure, so the script now ends with `exit 0`.

containerWorkdir defaulted to hostWorkspacePath. That default holds for an owned
container only because the create-time bind mount puts the host directory at that
exact path; attaching mounts nothing, so the two are independent facts. A host
path absent inside the container makes `docker exec --workdir` fail with an OCI
chdir error that surfaces in the pane as a bare "execvp failed". The preflight now
proves the directory exists inside the container and refuses at link time.

src/web/public/index.html

@@ -2856,6 +2856,11 @@
               <input type="text" id="dockerContainerName" placeholder="my-dev-box" pattern="[a-zA-Z0-9][a-zA-Z0-9_.-]+" autocomplete="off" autocapitalize="off" spellcheck="false">
               <span class="form-hint">Must be running already. <button type="button" class="btn-inline-check" id="dockerAdoptCheckBtn">Check container</button></span>
             </div>
+            <div class="form-row docker-adopt-only">
+              <label>Container Workdir</label>
+              <input type="text" id="dockerAdoptWorkdir" placeholder="/workspace" autocomplete="off" autocapitalize="off" autocorrect="off" spellcheck="false">
+              <span class="form-hint">A path that already exists <em>inside</em> the container. Adoption mounts nothing, so this need not match the host workspace path — leave blank to reuse it only if you mounted it there yourself.</span>
+            </div>
             <div class="form-row">
               <label>Case Name</label>
               <input type="text" id="dockerCaseName" placeholder="sandbox" pattern="[a-zA-Z0-9_-]+" autocomplete="off" autocapitalize="off" spellcheck="false">