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">

src/web/public/session-ui.js

@@ -2975,6 +2975,7 @@ Object.assign(CodemanApp.prototype, {
   async _dockerAdoptPreflight() {
     const statusEl = document.getElementById('dockerLinkStatus');
     const container = document.getElementById('dockerContainerName')?.value.trim();
+    const containerWorkdir = document.getElementById('dockerAdoptWorkdir')?.value.trim();
     const hostId = document.getElementById('dockerHostId').value.trim() || 'local';
     if (!container) {
       if (statusEl) statusEl.textContent = 'Enter a container name first.';
@@ -2986,7 +2987,7 @@ Object.assign(CodemanApp.prototype, {
     const probe = await this._apiJson('/api/docker-cases/adopt-preflight', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ hostId, container }),
+      body: JSON.stringify({ hostId, container, ...(containerWorkdir ? { containerWorkdir } : {}) }),
     });
     if (!statusEl) return;
     if (!probe) {
@@ -3009,6 +3010,7 @@ Object.assign(CodemanApp.prototype, {
     const hostId = document.getElementById('dockerHostId').value.trim() || 'local';
     const adopting = !!document.getElementById('dockerAdoptExisting')?.checked;
     const container = document.getElementById('dockerContainerName')?.value.trim() || '';
+    const adoptWorkdir = document.getElementById('dockerAdoptWorkdir')?.value.trim() || '';
     const image = document.getElementById('dockerImage').value.trim() || 'codeman/agent:base';
     const network = document.getElementById('dockerNetwork').value;
     const memory = document.getElementById('dockerMemory').value.trim();
@@ -3076,7 +3078,11 @@ Object.assign(CodemanApp.prototype, {
       const caseRes = await fetch(adopting ? '/api/cases/docker-adopt' : '/api/cases/docker-link', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify(adopting ? { name, hostId, hostWorkspacePath, container } : { name, hostId, hostWorkspacePath }),
+        body: JSON.stringify(
+          adopting
+            ? { name, hostId, hostWorkspacePath, container, ...(adoptWorkdir ? { containerWorkdir: adoptWorkdir } : {}) }
+            : { name, hostId, hostWorkspacePath }
+        ),
       });
       const caseData = await caseRes.json();
       if (caseData.success) {