ci: split Woodpecker pipelines by scope

- unit.yml: unit tests on every push and pull_request, all branches.
- code-runner.yml: PR to dev/main, gated on src/wrenn/code_runner/**
  or tests/test_code_runner_*.py; runs `make test-code-runner`.
- integration.yml: PR to dev/main, gated on src/** excluding
  src/wrenn/code_runner/**; runs `make test-integration`.

E2E pipelines require a src/** change, so docs/test-only PRs only
trigger the unit pipeline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.woodpecker/unit.yml

@@ -0,0 +1,11 @@
+# Unit tests — every push and pull_request, all branches.
+when:
+  - event: push
+  - event: pull_request
+
+steps:
+  unit-tests:
+    image: ghcr.io/astral-sh/uv:python3.13-bookworm
+    commands:
+      - uv sync --dev
+      - uv run pytest -m "not integration" -v