Make fable prove it read the staged context

[fcakyon]

#263 gave fable a pointer to a staged file, but nothing checked the reviewer read it: a skipped read printed the same marker as a full-context review.

• the staged file now carries a token the marker line must quote
• 32 random bits, absent from the pointer, so unguessable
• codex needs none, its file is concatenated into the prompt

+Confirmation token: ${randomUUID().slice(0, 8)}