fix(sandbox): prevent Windows PATH from poisoning docker exec (#13873)

* fix(sandbox): prevent Windows PATH from poisoning docker exec shell lookup

On Windows hosts, `buildDockerExecArgs` passes the host PATH env var
(containing Windows paths like `C:\Windows\System32`) to `docker exec -e
PATH=...`. Docker uses this PATH to resolve the executable argument
(`sh`), which fails because Windows paths don't exist in the Linux
container — producing `exec: "sh": executable file not found in $PATH`.

Two changes:
- Skip PATH in the `-e` env loop (it's already handled separately via
  OPENCLAW_PREPEND_PATH + shell export)
- Use absolute `/bin/sh` instead of bare `sh` to eliminate PATH
  dependency entirely

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: add braces around continue to satisfy linter

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(test): update assertion to match /bin/sh in buildDockerExecArgs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/agents/bash-tools.build-docker-exec-args.test.ts

@@ -76,7 +76,7 @@ describe("buildDockerExecArgs", () => {
       tty: false,
     });
 
-    expect(args).toContain("sh");
+    expect(args).toContain("/bin/sh");
     expect(args).toContain("-lc");
   });