docs(readme): rewrite root README for developer onboarding (#17347)

Drop every count-based statistic (test badge, package/schema/file/line/token
counts) and the sections that carried little value for a first-time reader
(FDE pitch, full scripts table, standalone roadmap/architecture/contributing
stubs). Lead with the five-minute loop, what one definition yields, why
mistakes don't ship, MCP, shipping, and a single "hack on the framework"
section.

Refresh the facts the old copy had drifted from: the package directory now
lists every published package and example in the workspace, the CLI list
matches packages/cli/src/commands, the toolchain line matches
packageManager, and the upgrade link points at the current docs page.


Claude-Session: https://claude.ai/code/session_016gFSzdPX4324QMRKCjccTc

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-sales