docs: add LICENSE, CONTRIBUTING, CHANGELOG, and NEWS files

Four new top-level docs to round out the project's public-facing
materials:

- LICENSE: full GPL-3 text, mirroring the header notice in
  org-drill.el (copied from emacs-wttrin's bundled license).
- CONTRIBUTING.md: bug-reporting guide, patch flow (fork → branch
  → tests → PR), development setup instructions wrapping the
  existing make targets, and testing/style notes pointing at the
  TDD discipline expected.
- CHANGELOG.md: keepachangelog format with an Unreleased section
  documenting every change since the fork — bug fixes by issue
  number, new infrastructure (CI, coverage, lint), structural
  refactors, and removals.
- NEWS: shorter user-facing summary highlighting the bugs most
  likely to have bitten existing users.

Closes the [#A] TODO 'Add CONTRIBUTING, CHANGELOG, NEWS, and a
standalone LICENSE file'.

1 files changed, 87 insertions, 0 deletions

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+# Contributing to org-drill
+
+Thanks for your interest. This is a small project; the contribution flow is informal.
+
+## Reporting a bug
+
+Open an issue at <https://github.com/cjennings/org-drill/issues> with:
+
+- The behavior you expected vs. what you saw
+- A minimal `:drill:`-tagged Org snippet that reproduces it (if applicable)
+- Your Emacs version (`M-x emacs-version`) and Org version (`M-x org-version`)
+- Anything in `*Messages*` that looks related
+
+If you have a stack trace, paste the full backtrace inside a fenced code block.
+
+## Sending a patch
+
+1. Fork the repo and branch from `main`.
+2. Write or update tests for whatever you're changing — see [Testing](#testing) below.
+3. Run `make test-unit` and `make compile` and confirm they pass.
+4. Open a PR. Conventional-commits-style commit messages are appreciated (`fix:`, `feat:`, `refactor:`, `test:`, `docs:`, `ci:`, `chore:`); see the existing `git log` for examples.
+
+CI will run the test matrix (Emacs 28.2, 29.4, snapshot), `make lint` (informational), `make compile`, and `make coverage` against your branch.
+
+## Development setup
+
+Requires [Cask](https://github.com/cask/cask) and Emacs 25.3+ with Org 9.6+ available.
+
+```sh
+# Install dependencies into a project-local .cask/ directory
+make setup
+
+# Run the unit-test suite
+make test-unit
+
+# Run a single test file
+make test-file FILE=test-org-drill-entry-empty-p.el
+
+# Run tests matching a name pattern
+make test-name TEST='*scheduler*'
+
+# Generate a coverage report (writes .coverage/simplecov.json)
+make coverage
+
+# Byte-compile org-drill.el (informational warnings)
+make compile
+
+# Run checkdoc + package-lint + elisp-lint (informational)
+make lint
+
+# Quick parens sanity check
+make validate-parens
+
+# List every available target
+make help
+```
+
+## Testing
+
+The project uses [ERT](https://www.gnu.org/software/emacs/manual/html_node/ert/) for unit tests. Tests live under `tests/` as `test-<topic>.el` files.
+
+When adding new code:
+
+- **Write a failing test first**, then implement until green. This isn't optional — `make coverage` is currently around 78%, and we want to keep it climbing.
+- For each new public function, cover three categories: normal cases (happy path), boundary cases (edge values, empty inputs, max/min), and error cases (invalid inputs, missing required state).
+- Mock at external boundaries (`current-time`, `read-key-sequence`, `read-string`, `org-id-get-create`). Don't mock internal helpers.
+- Slow tests (multi-second integration scenarios) should be tagged `:tags '(:slow)` so they're excluded from the default `make test-unit` runner.
+
+When fixing a bug:
+
+- Write a regression test that reproduces the bug **before** changing the implementation.
+- Verify the test fails on the unfixed code.
+- Apply the fix and verify the test passes.
+- Mention the upstream issue number in the commit message when applicable.
+
+The `robot/` directory contains end-to-end UI tests that drive a real Emacs frame via window-system key events. They require X11 and aren't part of `make test-unit`. Run them with `make robot` (or `make robot-all` for the full set) only on a graphical session.
+
+## Style notes
+
+- Lisp code should byte-compile cleanly with `make compile`. Existing source has known docstring debt that `make lint` will surface; new code should not add to it.
+- Function and variable names are `kebab-case` and prefixed with `org-drill-` (or `org-drill--` for internal).
+- Use `defcustom` for user-configurable settings, with a clear `:type` and a `:group` of `org-drill`.
+- Prefer `cl-letf`, `let`, and `unwind-protect` over global state mutation. Where global state is unavoidable (session objects, persisted matrices), keep its lifecycle explicit.
+
+## License
+
+By contributing you agree that your contributions are licensed under the same GPL-3.0-or-later as the rest of the project.