feat(scenario): step output, getStepOutput helper, export/import MCP tools, doc restructure

- add output column to ScenarioRunStepEntity; exec step return value is JSON-serialised and stored
- expose helpers.getStepOutput(order) in exec code; negative order is relative to current step
- add get_scenario_run, wait_for_scenario_run, export_scenario, import_scenario MCP tools
- move Architecture, Key descriptors, MCP tools, Scenario, Development docs to docs/
- delete CONTRIBUTING.md (replaced by docs/development.md)

docs/key-descriptors.md

@@ -0,0 +1,20 @@
+# Key descriptors
+
+Each file in `KEYS_DIR` is named `<keyId>.json` and takes one of two forms:
+
+**File key (EDS):**
+```json
+{
+  "keyFile": "path/to/key.dat",
+  "password": "secret"
+}
+```
+`keyFile` is resolved relative to `KEYS_DIR`.
+
+**Login + password:**
+```json
+{
+  "login": "user@example.com",
+  "password": "secret"
+}
+```