feat(phase-14/41): add mission.md briefing

phases/14-agent-engineering/41-workbench-for-real-repos/mission.md

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+# Mission - The Workbench on a Real Repo
+
+## Goal
+Run the same `/signup` validation task through a prompt-only pipeline and a workbench-guided pipeline against the same sample app, then emit a before/after comparison report a skeptic can read.
+
+## Inputs
+- `sample_app/` with `app.py` (no validation), `test_app.py` (one happy-path test), `README.md`, `scripts/release.sh` as forbidden-zone bait
+- Both pipelines fully scripted, no real LLM calls
+
+## Deliverables
+- `code/main.py` orchestrating both pipelines against the same fixture
+- `before-after-report.md` with the five outcomes table
+- `comparison.json` for downstream charting
+
+## Acceptance
+- `python3 code/main.py` exits zero
+- The report measures all five outcomes: tests actually ran, acceptance met, files outside scope, handoff quality, reviewer total
+- The workbench pipeline beats the prompt-only pipeline on at least four of the five
+
+## Out of scope
+- Plugging in a real LLM. The pipelines are scripted for reproducibility.
+- Tuning the model. The comparison holds the model constant by construction.
+
+## References
+- `docs/en.md` - full lesson
+- `code/main.py` - reference implementation
+- `outputs/skill-workbench-benchmark.md` - extracted skill