Mark Final SEP pages as historical records

Final SEPs are point-in-time records of the design as accepted. The
protocol can change after a SEP is finalized, so a Final SEP page can
drift from the current specification. SEP-2322 still documents
tasks/result and tasks/input_response even though SEP-2663 removed
them (#3142).

The SEP generator now injects a notice at the top of every Final SEP
page pointing readers to the current specification. The SEP guidelines
now state that Final SEPs are not updated after finalization and that
the current specification is authoritative.

Closes #3142

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XY2THv6f61qBZdC8j7uPZP