Version each model description and let implementations cite the version they follow #13

Model descriptions will change: rarely, but wording gets clarified, and now and then the model or the calibration changes. Each implementation needs to say which version it follows, and a reader of the PDF needs to see the version at a glance. The buffer-stock description from #5 is at version 0.1 (draft). This issue settles the scheme before the first version is tagged.

Agreed scheme

• MAJOR.MINOR, numbered separately for each model description. A new major version means implementations must re-run, because the model, the calibration or the required results changed. A new minor version is a touch-up, such as clarified wording, a corrected typo or an added note, and leaves the results alone.
• 0.x is a draft; 1.0 is the working group's release. While a description is at 0.x, every change raises the minor number, and its changelog entry says whether implementations need to re-run. From 1.0, the major number carries that.

Plan

• One source. version, status (draft or released) and date go in the front matter of models/<model>/model-description.md. The subtitle drops "version 0.1 (draft)".
• A version box at the top right of the PDF, showing the version, status and date. templates/plain_latex_wide/ already takes template options and loads tcolorbox, so the box is a small template change. The web page from Publish each model description as a web page alongside its PDF #12 shows the same box, which also carries the AI-disclosure and verification indicators from Disclose AI assistance and record human verification #14.
• A Git tag and GitHub release for each version, such as buffer-stock/v0.1, with the PDF attached. That gives a permanent link to exactly the version an implementation follows.
• A changelog for each model, models/<model>/CHANGELOG.md. Each entry gives the version, the date, what changed and whether implementations need to re-run.
• Implementations cite the version in metadata.yml as model_description_version, with model_description linking to the tag. The HARK example already has both fields; its link currently points at main. The site's library table shows the version in each slot, for example "HARK · v0.1".
• The rules go in a root CONTRIBUTING.md, next to the disclosure rules from Disclose AI assistance and record human verification #14.
• Optional: a CI check that a pull request editing model-description.md also changes its version or its changelog.

The first tag, buffer-stock/v0.1, should be made once the PDF carries the box, so the first tagged PDF already shows it. This is planned to land together with #12, after #5 merges.