docs: refocus README on users, add quick_start + tutorial, fill CONTRIBUTING

- README.md: pruned developer-oriented sections (Sphinx setup, Qt
  Creator workflow, VSCode debugging, release procedure, AppImage
  Wayland note) and replaced them with a user-facing layout: pre-built
  releases pointer, quick start, manual install, troubleshooting,
  licence.
- CONTRIBUTING.md: absorbed the developer content (debugging in VSCode,
  Qt GUI regen, Sphinx build, validation suite — batch + GUI variants,
  cross-distrib check, release procedure).
- doc/quick_start.md: 5-minute path from install to a passing test,
  in batch mode and in the GUI.
- doc/tutorial.md: guided walk-through against a small calc.py
  module — check, py_func, expected_result, $(...) expansion, group,
  let, condition, report (with the mkdir reminder), context_id.
- CLAUDE.md: subprocess API contract, bins.py, report-exporter
  plugin section, packaging matrix (wheel / PyInstaller / Flatpak /
  .deb work-in-progress), refreshed recent-fixes list. README/CLAUDE
  validation command no longer carries the spurious "-l" flag (which
  is GUI-only and a no-op in batch).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

doc/quick_start.md

@@ -0,0 +1,66 @@
+# Quick start
+
+Five minutes from zero to a passing test.
+
+## Install
+
+From a checkout of the repository:
+
+```sh
+./run.sh --version    # Linux
+run.bat               # Windows cmd
+```
+
+The wrapper creates a Python virtual environment on first run and verifies
+testium starts. If you prefer a manual install, see the README.
+
+## Your first test
+
+Create `hello.tum`:
+
+```yaml
+main:
+    name: hello world
+    steps:
+        - check:
+            name: 1 + 1 makes 2
+            values:
+                - <| 1 + 1 == 2 |>
+```
+
+Run it in batch mode:
+
+```sh
+./run.sh -b -- hello.tum
+```
+
+You should see something like:
+
+```
+-----> step "1 + 1 makes 2" started
+Check passed
+<----- step "1 + 1 makes 2" finished: PASS
+Test run success.
+```
+
+Replace `==` with `!=` and re-run — the step now ends with **FAIL** and
+the process exits with code 1.
+
+## Open it in the GUI
+
+```sh
+./run.sh hello.tum
+```
+
+The test tree appears in the left panel; click *Run test* in the toolbar.
+Each item turns green or red live as it executes. Use `F1` on a selected
+item to open its detail panel.
+
+## Where to go next
+
+* [`doc/tutorial.md`](tutorial.md) — a guided walk-through of the most
+  common test items (`py_func`, `let`, `group`, `condition`, `report`).
+* [`doc/examples/`](examples/) — runnable `.tum` snippets covering one
+  feature each.
+* [`doc/manual/testium_manual.pdf`](manual/testium_manual.pdf) —
+  full reference manual.