fix(cursor-local): resolve sandbox agent installs from cursor bin (#5686)

> _Stacked on top of #5685 (Harden remote sandbox runtime). Diff against
master includes commits from earlier PRs in the stack — review focuses
on the new commit only._

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The cursor-local adapter wraps the Cursor Agent CLI so a Paperclip
workflow can drive it inside a sandbox
> - When the adapter runs in a remote sandbox, the Cursor Agent CLI
installs under `$HOME/.local/bin/cursor-agent` (or wherever
`$XDG_BIN_HOME` points), not on the global PATH
> - The existing post-install resolution assumed `cursor-agent` would
resolve via the sandbox's login shell PATH after `npm install -g`, which
fails on sandboxes where the install lands in a user-prefixed directory
that isn't on PATH at probe time
> - This pull request resolves the agent CLI from the cursor binary's
own directory (`dirname "$(command -v cursor)"`) so the install probe
and execute path agree on a real binary location
> - The benefit is that cursor-local works correctly on any sandbox
provider where `npm install` lands in a user-prefixed directory

## What Changed

- `packages/adapters/cursor-local/src/server/remote-command.ts`: resolve
the cursor-agent binary from the cursor bin directory after install,
instead of relying on PATH.
- `packages/adapters/cursor-local/src/server/test.ts`: corresponding
probe tweak.
- `packages/adapters/cursor-local/src/server/test.test.ts` (new) +
`remote-command.test.ts`: focused coverage that exercises the install +
resolve path against a sandbox runner that places the binary in a
user-prefixed directory.

## Verification

- `pnpm exec vitest run --no-coverage
packages/adapters/cursor-local/src/server/test.test.ts
packages/adapters/cursor-local/src/server/remote-command.test.ts
packages/adapters/cursor-local/src/server/execute.test.ts`

All passing locally.

## Risks

- Local cursor-local runs are unaffected — the resolution change only
kicks in for the sandbox install path.
- Low risk; isolated to one adapter.

## Model Used

- Provider: Anthropic
- Model: Claude Opus 4.7 (1M context)
- Capabilities used: tool use (Read/Edit/Bash), no code execution beyond
local repo commands

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots — N/A, no UI change
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Paperclip <noreply@paperclip.ing>

packages/adapters/cursor-local/src/server/remote-command.test.ts

@@ -44,6 +44,52 @@ printf '%s\\n' ok
 }
 
 describe("prepareCursorSandboxCommand", () => {
+  it("prefers the Cursor installer bin directory when the default agent entrypoint is installed there", async () => {
+    const root = await fs.mkdtemp(path.join(os.tmpdir(), "paperclip-cursor-remote-command-cursor-bin-"));
+    const systemHomeDir = path.join(root, "system-home");
+    const managedHomeDir = path.join(root, "managed-home");
+    const remoteWorkspace = path.join(root, "workspace");
+    const cursorAgentPath = path.join(systemHomeDir, ".cursor", "bin", "agent");
+    await fs.mkdir(remoteWorkspace, { recursive: true });
+    await writeFakeAgent(cursorAgentPath);
+
+    try {
+      const result = await prepareCursorSandboxCommand({
+        runId: "run-remote-command-cursor-bin",
+        target: {
+          kind: "remote",
+          transport: "sandbox",
+          shellCommand: "bash",
+          remoteCwd: remoteWorkspace,
+          runner: createLocalSandboxRunner(),
+          timeoutMs: 30_000,
+        },
+        command: "agent",
+        cwd: remoteWorkspace,
+        env: {
+          HOME: managedHomeDir,
+          PATH: "/usr/bin:/bin",
+        },
+        remoteSystemHomeDirHint: systemHomeDir,
+        timeoutSec: 30,
+        graceSec: 5,
+      });
+
+      expect(result.command).toBe(cursorAgentPath);
+      expect(result.preferredCommandPath).toBe(cursorAgentPath);
+      expect(result.remoteSystemHomeDir).toBe(systemHomeDir);
+      expect(result.addedPathEntry).toBe(path.join(systemHomeDir, ".local", "bin"));
+      expect(result.env.PATH?.split(":").slice(0, 2)).toEqual([
+        path.join(systemHomeDir, ".local", "bin"),
+        path.join(systemHomeDir, ".cursor", "bin"),
+      ]);
+      expect(result.env.PATH).not.toContain(path.join(managedHomeDir, ".cursor", "bin"));
+      expect(result.env.PATH).not.toContain(path.join(managedHomeDir, ".local", "bin"));
+    } finally {
+      await fs.rm(root, { recursive: true, force: true });
+    }
+  });
+
   it("keeps probing the original sandbox home after managed HOME overrides", async () => {
     const root = await fs.mkdtemp(path.join(os.tmpdir(), "paperclip-cursor-remote-command-"));
     const systemHomeDir = path.join(root, "system-home");
@@ -79,7 +125,10 @@ describe("prepareCursorSandboxCommand", () => {
       expect(result.preferredCommandPath).toBe(systemAgentPath);
       expect(result.remoteSystemHomeDir).toBe(systemHomeDir);
       expect(result.addedPathEntry).toBe(path.join(systemHomeDir, ".local", "bin"));
-      expect(result.env.PATH?.split(":")[0]).toBe(path.join(systemHomeDir, ".local", "bin"));
+      expect(result.env.PATH?.split(":").slice(0, 2)).toEqual([
+        path.join(systemHomeDir, ".local", "bin"),
+        path.join(systemHomeDir, ".cursor", "bin"),
+      ]);
       expect(result.env.PATH).not.toContain(path.join(managedHomeDir, ".local", "bin"));
     } finally {
       await fs.rm(root, { recursive: true, force: true });