README.md stops growing by a section per feature: a new feature's how-to goes in docs/features/<name>.md #6380

Part of #5930 (append-only registries become one file per entry). README.md was changed by 155 landed pulls in a week and is 2,329 lines long. Each pull appends a section, and they collide.

Done:

• [ ] docs/features/ exists, with one short README.md there saying a feature's how-to goes in its own file.
• [ ] A test fails when README.md has more lines than a cap set at today's count. Its message says to write docs/features/<name>.md instead.
• [ ] CLAUDE.md and AGENTS.md say so in one line each, where they speak of the README.
• [ ] Existing sections stay where they are; moving them is not this issue.

Files: docs/features/README.md, a test, CLAUDE.md, AGENTS.md.

Before pushing: bun x tsc --noEmit and the tests you add or touch. Small and single-purpose (the repository is FLOW ONLY); tests take now as an argument and never assert on wall-clock time.