docs: README follows agent-skills' layout, with a lifecycle hero

[ttncode]

What this changes

The README follows the layout of addyosmani/agent-skills: a bold tagline, a hero figure, a command table with a key principle per row, what a generated project gets, adapters and services, how it works, project structure, why scaffold, documentation, contributing and a team table. Detail that already lives in docs/ is linked instead of repeated, so the README keeps only what a first-time reader needs.

The hero, docs/diagrams/scaffold-lifecycle.png, shows a client project from scaffold new to a running release, with scaffold update and the nine-task contract. It is drawn with the diagram-design skill in the same default skin as the other diagrams, with its .html source beside it. It is a PNG rather than an SVG because GitHub shows README images without the web fonts an SVG imports. docs/README.md lists it in the diagrams table.

No code or behaviour changes.

How it was verified

• mise run lint: passes (also ran as the pre-commit hook, with gitleaks: no leaks found).
• mise run test-runner: exit 0. Unit lane 201 of 201, integration lane 69 of 69, nothing skipped.
• bats tests/documentation.bats: 5 of 5. Every path named, every relative link and image, and every ADR cited in the new README resolves.

Checklist

• mise run lint passes
• mise run test-runner passes
• New behaviour has a test that fails without the change: not applicable, no behaviour changes
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes