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/index.ts

@@ -104,5 +104,5 @@ Notes:
 - Sessions are resumed with --resume when stored session cwd matches current cwd.
 - Paperclip auto-injects local skills into "~/.cursor/skills" when missing, so Cursor can discover "$paperclip" and related skills on local runs.
 - Paperclip auto-adds --yolo unless one of --trust/--yolo/-f is already present in extraArgs.
-- Remote sandbox runs prepend "~/.local/bin" to PATH and prefer the installed "~/.local/bin/agent" or "~/.local/bin/cursor-agent" entrypoint when the default Cursor command is requested, so standard E2B-style installs do not need hardcoded absolute command paths.
+- Remote sandbox runs prepend "~/.cursor/bin" and "~/.local/bin" to PATH and prefer the installed absolute entrypoint from one of those directories when the default Cursor command is requested, so installer-managed sandbox leases do not need hardcoded command paths.
 `;