backfill-feature - characterize existing code before changing it

When you reach to modify code that has no ADR or spec, you're working blind: you don't know what the original author intended, what constraints were considered, or what behaviour the rest of the system depends on. /backfill-feature builds that picture before the change, so the change lands on solid ground.

It writes three things in order:

1. A reverse-engineered ADR ("here's what's true today, here's why I think it ended up this way, here's what's load-bearing").
2. A Gherkin spec capturing current behaviour.
3. Characterization tests that lock current behaviour green.

After backfill, /plan-change runs to layer the actual change on top of the (now-documented, now-tested) baseline.

When to use