improve: add AGENTS.md and CI workflow

[Coding-Dev-Tools]

Automated improvement by dev-engineer

• Adds AGENTS.md with agent orientation for this repo
• Adds .github/workflows/ci.yml to run pytest on push/PR

[Coding-Dev-Tools]

🔒 Sentinel review: this PR cannot merge at this time.

Gates:
• Age: not >6h yet.
• Contributors: 1 distinct contributor; needs >=3.
• CI: no checks reported on the branch.
• Quality: em-dash characters are corrupted to “—” in multiple blog meta/description fields — encode as UTF-8 source/em-dash directly.

Please address and rerun CI; sentinel will re-evaluate once the branch checks appear.

[Coding-Dev-Tools]

[Blocked — branch protection + conflict] Same test/enforce_admins branch-protection block as #63 (no workflow emits the required test check), and this branch conflicts with main. Needs a rebase plus the repo-config fix before it can merge.