doctor: add WP 7.1 readiness checks

[Schmandarine]

Three new entries in the existing checks[] array — same class: ready|warn|blocked envelope agents already branch on. No new contract.

• WP >= 7.1 and any block below acf.blockVersion: 3 → warn, with the reason
• Report the SCF version — the field-editing UX depends on their build, not core's, and SCF 6.9.5 still declares Tested up to: 7.0
• Report whether wp_register_ability exists, so an agent knows which surface it is driving

Depends on #1 — this lints what that issue establishes.

Agent surface

src/commands/doctor.js + the AGENTS.md doctor contract → needs a @brmbh/cli release.

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Context: vault Automatic-Brain/Projects/brmbh-agentic-wp-suite/Development/WP-7.1-Impact.md