flan/docs/handoffs/HANDOFF-tidy.md

Tidying the repository root

The root had thirty-odd files in it, nine of them session reports sitting at the same level as the language specification, and a README is about to point a newcomer at that. This is the plan and, below it, what actually happened.

The plan

1. git mv the nine HANDOFF-*.md into docs/handoffs/, and ../DISCUSS.md, ../BUILT.md, ../PORTING.md, ../SPIKE-GENERICS.md, ../REFERENCES.md and ../overview.md into docs/. These are written once and read occasionally.
2. Leave plan.org, spec-memory.md, spec-conditions.md, NEXT.md, the .flan scratch programs and the build files where they are: they are cited by bare filename from source comments in dozens of places, and moving them would be a large rewrite for no gain.
3. Rewrite every citation of a moved file, zone by zone — a flat substitution is wrong, because a citation inside docs/ should stay bare while the same citation in lib/ needs the docs/ prefix.
4. Verify by resolving, not by grepping: for every cited path in the repository, check it exists relative to the citing file or to the root.
5. Write docs/README.md saying which of these documents are still true.

What happened

(filled in as the work lands)