feat: add canonical run-tests runner (Closes #473)

Merge PR #476 — root-level run-tests.sh full-validation runner (Closes #473).

docs/developer-testing-guidelines.md

@@ -13,6 +13,25 @@ credentials.** Every test mocks the HTTP client and the keychain/auth lookup.
 
 ## 1. Standard test commands
 
+### Canonical runner: `./run-tests.sh`
+
+The canonical full-validation command is the root-level runner. It invokes the
+project virtualenv interpreter and passes any extra arguments straight through
+to `pytest`:
+
+```bash
+# Full validation
+./run-tests.sh
+
+# Focused validation (extra args forward to pytest)
+./run-tests.sh tests/test_mcp_server.py -q
+```
+
+`run-tests.sh` runs `venv/bin/python -m pytest "$@"` and fails with a clear
+setup message if the virtualenv Python is missing (so a session never silently
+falls back to the wrong interpreter). The explicit `venv/bin/python -m pytest`
+forms below remain valid and equivalent.
+
 The test suite needs the project virtualenv (it provides the MCP SDK):
 
 ```bash