docs: align Go requirements with source modules - #58

Documentation-only PR that updates Go version requirements for seid (1.25.6) and seictl (1.26.0), hardens the Go install snippet with a checksum verification step, and clarifies that prebuilt/Docker paths need no Go. No correctness or security blockers found; the notes below concern maintainability of hardcoded values, a PATH-shadowing gap the new Note diagnoses but doesn't fix, and consistency of the GOTOOLCHAIN guidance.