fix(write-tests): use reference_branches for source repo; fix spec path

AR-27b filters clone pre_actions when agent-repo is active, so the source
repo was never cloned and the spec was unreachable. Use reference_branches
instead: the source repo is cloned read-only to /workspace/reference/main/
by agent-repo/v1/init.sh.

Both write_tests and review_coverage nodes now declare:
  reference_branches:
    - repo_url: "{{ repo_url }}"
      branch: main
      name: main

Prompts updated to read from /workspace/reference/main/{{ spec_file }}
and write output to /workspace/project/{{ test_output_dir }}.

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

workflows/write-tests.yaml

@@ -48,12 +48,16 @@ params:
 nodes:
   write_tests:
     name: "Write tests from spec"
+    reference_branches:
+      - repo_url: "{{ repo_url }}"
+        branch: main
+        name: main
     prompt: |
       You are writing a test suite from a spec file. Tests are the contract — they will
       be run against an agent's implementation. Write them now, before the implementation exists.
 
       ## Spec file
-      Read the spec at `{{ spec_file }}` in the cloned repo at `/workspace/project/`.
+      Read the spec at `/workspace/reference/main/{{ spec_file }}`.
 
       {% if task_description %}
       ## Additional context
@@ -65,7 +69,7 @@ nodes:
       2. For each requirement, write at least one test function named `test_<req_id>_<short_description>`.
       3. Cover edge cases explicitly — empty inputs, boundary values, error paths.
       4. Add property-based tests (Hypothesis) where the requirement involves ranges or invariants.
-      5. Write tests to `{{ test_output_dir }}` — one file per logical group (mirrors spec sections).
+      5. Write tests to `/workspace/project/{{ test_output_dir }}` — one file per logical group (mirrors spec sections).
       6. Use `{{ test_framework }}` conventions. Tests should fail immediately (no implementation exists).
       7. Do NOT write any implementation code. Only test files.
 
@@ -82,11 +86,15 @@ nodes:
   review_coverage:
     name: "Review test coverage"
     depends_on: [write_tests]
+    reference_branches:
+      - repo_url: "{{ repo_url }}"
+        branch: main
+        name: main
     prompt: |
       You are reviewing the coverage of a freshly written test suite against its spec.
 
       ## Spec file
-      Read the spec at `{{ spec_file }}` in the cloned repo at `/workspace/project/`.
+      Read the spec at `/workspace/reference/main/{{ spec_file }}`.
 
       ## Test summary written by the previous agent
       <<ARTIFACT:write_tests:output>>