v0.7.7 - HTTP pre-flight for routine writes

gsc_submit_sitemap and indexnow_submit/bulk_submit now HTTP pre-flight the resource they depend on (the sitemap URL; the IndexNow key file) before forwarding upstream, using the branded SEOMonster UA. A 404/unreachable sitemap or a missing/mismatched key file now returns INVALID_INPUT with remediation instead of a false success (the upstreams accept blindly and fail silently downstream). Adds a skip_preflight escape hatch. No new tools or prompts (52 tools / 13 prompts). Closes FEEDBACK §20 §1b/§3c/§3d. See CHANGELOG.md.