Docs: unify skeleton README

[ohmyfelix]

Scope

• Replace stale onboarding with concise API Router skeleton setup and repository-grounded guidance.
• Avoid duplicate dependency installation by using make setup after Composer project creation.
• Document the mismatched local-config ignore path and the current route compatibility blocker.

Verification

• git diff --check passed.
• Central README evaluator: 7 passed, 0 failed, 7 semantic/runtime checks unassessed.
• make tests: OK (1 test).
• Clean-clone smoke: setup and server startup executed; /api/ping returned 404 because api-router 7.x discovers PHP attributes while the skeleton controllers use legacy docblock annotations.