Enforce the dev branch lifecycle

[Milky2018]

Summary

• make dev the development integration branch and reserve main for stable releases
• forbid reuse of merged or closed PR source branches
• require open-PR coverage or an origin/dev ancestry check before reporting delivery
• require merged source branches to be retired

Validation

• git diff --check
• pre-commit MoonBit checks and 68 script tests

[Milky2018]

Closing because the branch model was specified incorrectly. Development commits belong directly on the long-lived dev branch; dev is periodically merged into main.