cmd/loop: bless text-only session updates

Add an env-gated bless mode to recorded-session replay so
CLI-only text changes can refresh fixtures without a live
recording pass. The replay still uses the recorded gRPC
traffic, stdin, and environment, and it only rewrites
stdout, stderr, and run_error after the command preserves
the recorded success or failure shape.

Keep the updater strict by refusing to bless sessions when
replay leaves recorded gRPC events unconsumed, and add
focused tests for the rewrite rules. Document the bless
workflow next to the session fixtures, including the need
for -count=1 so the Go test cache does not skip updates.

Author: starius