ci: let Dependabot pull requests skip the description template - #7

What

The Conventions check skips the description template for pull requests opened by Dependabot (dependabot[bot], account type Bot). Their title is still checked as Conventional Commits, and every other check runs as before. CONTRIBUTING.md mentions the exemption.

Why

Dependabot writes its own description, so Conventions failed every update it opened (#2: sections What, Tested and Checklist missing). With Conventions required on main, those updates could never be merged. Usernames cannot contain brackets, so no person can claim the exemption.

Tested

• New tests in tools/tests/test_check_conventions_pr_template.py, driven through a GitHub event file:
  • a Dependabot pull request passes;
  • its title is still checked;
  • the same description fails for a user, for a user named like the bot, for another bot and for a missing user.
• Deliberate mistakes, each caught by the tests:
  • exempt everyone;
  • exempt no one;
  • match the name only, without the account type;
  • skip the title check.
• The real event of ci: bump actions/github-script from 8.0.0 to 9.0.0 in the actions group across 1 directory #2: the checker on main reports the 3 template errors seen in CI; the changed checker passes it.
• python tools/check_conventions.py --base main: all checks passed. The checker's unit tests pass on Windows.

Checklist

• One topic, based on current main
• Code follows docs/dev/CONVENTIONS.md; unimplemented paths throw
• Generic behaviour, not specific to one title
• Performance change: behind an APS5_NO_* switch, documented in docs/dev/ENVIRONMENT.md, with a test that fails on a deliberate mistake
• No game files, executables, dumps or frame captures in the repository or in this pull request
• Depends on: none
• Fixes: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes