fix(channels,sandbox): land hard breakage cluster from reviewed PR bases

Lands reviewed fixes based on #25839 (@pewallin), #25841 (@joshjhall), and #25737/@25713 (@DennisGoldfinger/@peteragility), with additional hardening + regression tests for queue cleanup and shell script safety.

Fixes #25836
Fixes #25840
Fixes #25824
Fixes #25868

Co-authored-by: Peter Wallin <pwallin@gmail.com>
Co-authored-by: Joshua Hall <josh@yaplabs.com>
Co-authored-by: Dennis Goldfinger <dennisgoldfinger@gmail.com>
Co-authored-by: peteragility <peteragility@users.noreply.github.com>

src/agents/sandbox/fs-bridge.test.ts

@@ -77,10 +77,22 @@ describe("sandbox fs bridge shell compatibility", () => {
     const executables = mockedExecDockerRaw.mock.calls.map(([args]) => args[3] ?? "");
 
     expect(executables.every((shell) => shell === "sh")).toBe(true);
-    expect(scripts.every((script) => script.includes("set -eu;"))).toBe(true);
+    expect(scripts.every((script) => /set -eu[;\n]/.test(script))).toBe(true);
     expect(scripts.some((script) => script.includes("pipefail"))).toBe(false);
   });
 
+  it("resolveCanonicalContainerPath script is valid POSIX sh (no do; token)", async () => {
+    const bridge = createSandboxFsBridge({ sandbox: createSandbox() });
+
+    await bridge.readFile({ filePath: "a.txt" });
+
+    const scripts = mockedExecDockerRaw.mock.calls.map(([args]) => args[5] ?? "");
+    const canonicalScript = scripts.find((script) => script.includes("allow_final"));
+    expect(canonicalScript).toBeDefined();
+    // "; " joining can create "do; cmd", which is invalid in POSIX sh.
+    expect(canonicalScript).not.toMatch(/\bdo;/);
+  });
+
   it("resolves bind-mounted absolute container paths for reads", async () => {
     const sandbox = createSandbox({
       docker: {

src/agents/sandbox/fs-bridge.ts

@@ -305,7 +305,7 @@ class SandboxFsBridgeImpl implements SandboxFsBridge {
       "done",
       'canonical=$(readlink -f -- "$cursor")',
       'printf "%s%s\\n" "$canonical" "$suffix"',
-    ].join("; ");
+    ].join("\n");
     const result = await this.runCommand(script, {
       args: [params.containerPath, params.allowFinalSymlink ? "1" : "0"],
     });