## Implementation Plan: Fix Zip Service 100% Failure Rate

### Changes Required

#### 1. Remove spurious `_guard_writable` from `export_to_dir`

**File:** `devplacepy/project_files.py`  
**Action:** Delete line 688 (`_guard_writable(project_uid)`) inside `export_to_dir`.  
**Rationale:** `export_to_dir` is a read-only operation; the guard incorrectly blocks export for read-only projects, causing all zip, fork, and container jobs to fail.

#### 2. Fix stale `STAGING_DIR` import in CLI

**File:** `devplacepy/cli/jobs.py`  
**Actions:**
- Change line 10 from  
  `from devplacepy.services.jobs.zip_service import STAGING_DIR`  
  to  
  `from devplacepy.config import ZIP_STAGING_DIR`
- Change line 15 (where `STAGING_DIR` is used) to reference `ZIP_STAGING_DIR`.

**Rationale:** The constant was renamed in commit `1294c95`; the CLI import is stale and causes `ImportError` on `devplace zips prune` / `devplace zips clear`.

### Verification & Definition of Done

- [ ] Both file changes applied as described above.
- [ ] Project builds and installs without errors even in a restricted environment:
      `pip install -e '.[dev]' -q --break-system-packages`
- [ ] Linter passes with no new violations:
      `pip install -q ruff && ruff check .`
- [ ] All unit tests pass:
      `make test-unit` (or `python3 -m pytest tests/unit -x -q` if Makefile absent)
- [ ] (Optional but recommended) Add a unit test in `tests/unit/services/jobs/zip_service.py` that creates a project with `read_only=True` and asserts a successful zip job completes – to prevent regression.