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/schemas.ts

@@ -834,6 +834,14 @@ export const DockerAdoptPreflightSchema = z.object({
     .min(2)
     .max(128)
     .regex(/^[a-zA-Z0-9][a-zA-Z0-9_.-]+$/, 'Invalid container name'),
+  /** Optional: also verify this path exists INSIDE the container. */
+  containerWorkdir: z
+    .string()
+    .min(1)
+    .max(2000)
+    .regex(/^\//, 'Container workdir must be absolute')
+    .regex(NO_SHELL_META, 'Invalid characters in container workdir')
+    .optional(),
 });
 
 export const DockerExportSchema = z.object({