ENG-783 - A ticket's key has one name

[czpython]

Residue after #129: the column, both wire responses, and every frontend consumer
say ticket_key. Two places still said ref, carrying the same value.

TicketCell took ticketRef?: string | null plus a required fallback, and
ended on {ticketRef ?? fallback}. Its one caller passes wi.ticketKey, which is
a required string backed by a non-null column — so the optionality, the ??
tail, and fallback itself were unreachable. The prop is now ticketKey: string
and the dead path is gone.

Build's input carried three ticket fields. ticket_title and ticket_url
were write-only: dispatch set them, run_multistep declared them, and nothing
read them — BuildPromptContext has no title field, no template names either, and
the only reads repo-wide were two assertions pinning the wiring. ticket_ref was
read, but dispatch assigned it item.ticket_key unchanged and the workflow
already holds the row. All three are off the input; the prompt context renders
ticket_ref from self.subject.ticket_key.

BuildPromptContext.ticket_ref and the templates are unchanged — that field is
the prompt-facing reference, and it now reads the row instead of a frozen copy.

Renaming ticket_ref → ticket_key on the input was the obvious fix and the
wrong one: it buys one canonical spelling and leaves the input still carrying
ticket fields, so the next spelling question costs another drain. Removing the
fields settles it.