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>

CONTRIBUTING.md

@@ -45,7 +45,7 @@ For existing files, keep the header that is already there.
 3. Commit with a clear message (one logical change per commit).
 4. Make sure the validation suite still passes:
    ```
-   ./run.sh -b -l mon_log.log -- test/validation/main.tum
+   ./run.sh -b -- test/validation/main.tum
    ```
 5. Open a pull request against `main`.
 
@@ -56,6 +56,105 @@ For existing files, keep the header that is already there.
 - Add or update tests in `test/validation/` for new test items or behaviours
 - Update `CLAUDE.md` and the Sphinx manual for user-visible changes
 
+## Development
+
+### Debugging in VSCode
+
+The recommended workflow:
+
+1. Add a debug configuration to `.vscode/launch.json`:
+   ```json
+   {
+       "configurations": [
+           {
+               "name": "Python : testium",
+               "type": "python",
+               "request": "launch",
+               "program": "${workspaceFolder}/src/testium",
+               "console": "integratedTerminal",
+               "args": ["-g"],
+               "justMyCode": true
+           }
+       ]
+   }
+   ```
+2. Install `debugpy` in the venv: `python -m pip install debugpy`.
+3. Open the *Run and Debug* tab and press play. testium starts; load and
+   run a `.tum` file. Set breakpoints where you want to investigate.
+
+### Qt GUI modification
+
+UI files (`*.ui`) are edited in **Qt Creator**. After editing, regenerate
+the corresponding Python and resource files:
+
+```sh
+scripts/qt_generate.sh
+```
+
+Icons come from <https://github.com/free-icons/free-icons>.
+
+### Sphinx documentation
+
+```sh
+pip install sphinx linuxdoc
+doc/manual/sphinx/build_doc.sh
+```
+
+PDF generation requires `texlive`:
+
+```sh
+sudo apt install texlive-full
+```
+
+### Validation suite
+
+Batch mode (CI-friendly, headless):
+
+```sh
+./run.sh -b -- test/validation/main.tum
+```
+
+GUI mode (loads the suite, click *Run* to execute and inspect the tree):
+
+```sh
+./run.sh test/validation/main.tum
+```
+
+GUI run-and-close (executes the suite, then closes):
+
+```sh
+./run.sh -r -- test/validation/main.tum
+```
+
+Subset run via the `items` define (works in any mode):
+
+```sh
+./run.sh -b -d "items=['parallel','common']" -- test/validation/main.tum
+```
+
+### Cross-distribution check
+
+`package/deb/test_distro.sh` spins up a Docker/Podman container of the
+target image, installs the expected system Python deps via apt (with
+pip fallback for what is missing), installs the testium wheel and runs
+the validation suite end-to-end. Currently green on `debian:bookworm`,
+`debian:trixie`, `ubuntu:24.04`.
+
+```sh
+./package/deb/test_distro.sh debian:trixie
+```
+
+## Release procedure
+
+1. Update `release_note.txt`.
+2. Bump the version in `src/VERSION`.
+3. Make sure the documentation is up to date — rebuild with
+   `doc/manual/sphinx/build_doc.sh` if needed.
+4. Push and tag the commit with the new version.
+5. Build the binary release: `package/pyinstaller/build.sh`.
+6. Run the validation suite against each generated binary.
+7. Confirm all validation results are green before publishing.
+
 ## Reporting security issues
 
 Please do **not** report security vulnerabilities through public GitHub