Add write-code-simple workflow: narrow airouter single-test impl

Single-node airouter workflow for one-requirement/one-test tasks.
Optional test_target param focuses the agent on a specific pytest
node ID. Cost-optimised: labels=[airouter], cost_efficiency≥9.

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

workflows/write-code-simple.yaml

@@ -0,0 +1,61 @@
+name: write-code-simple
+version: 1
+description: >
+  Single-node airouter workflow for narrow implementation tasks: write just enough
+  code to make one specific test pass. Optimised for cost and speed — use for
+  single-requirement, single-file, or single-test work items.
+
+  Hard cap: 3 concurrent airouter tasks. No web search available in container.
+
+params:
+  required:
+    task_description:
+      type: string
+      description: "What to implement — be specific about which test must pass"
+    project_id:
+      type: string
+      description: "Project identifier for task grouping"
+    repo_url:
+      type: string
+      description: "Git repo URL for the agent to clone"
+    agent_repo_url:
+      type: string
+      description: "Agent repo URL for branch-per-task persistence"
+  optional:
+    test_target:
+      type: string
+      default: ""
+      description: "Specific test file or node ID to run (e.g. tests/test_foo.py::test_bar). Leave blank if specified in task_description."
+    harness:
+      type: string
+      default: "code-airouter-repo/v1"
+      description: "Harness to use (default: code-airouter-repo/v1)"
+    space_id:
+      type: string
+      default: "default"
+      description: "Space identifier"
+
+nodes:
+  agent:
+    name: "agent"
+    prompt: |
+      {{ task_description }}
+
+      {% if test_target %}
+      ## Target test
+      Your work is complete when this test passes:
+      ```
+      python -m pytest {{ test_target }} -v
+      ```
+      Do not modify the test file. Write only the source code needed to make it pass.
+      {% endif %}
+    harness: "{{ harness }}"
+    requirements:
+      labels: [airouter]
+      min_scores:
+        cost_efficiency: 9
+    on_success:
+      - type: commit_pr
+        params:
+          repo_url: "{{ repo_url }}"
+          agent_repo_url: "{{ agent_repo_url }}"