Every agent call names the agent that made it

[czpython]

AgentCall.agent was nullable, AgentCallResponse.agent optional, and the
response's label fell back to the string "Agent" when it was missing.
Nothing could produce that row.

The one production writer is AgentCall.start(..., agent=self.id)
(agents.py), where self is an Agent. An
explicit id registers in __post_init__; otherwise __set_name__ stamps it
from the attribute name. Even seed_call takes agent as a required
positional. So the | None, the nullable column and the "Agent" fallback
modelled an absence with no producer — and #142 turning label into a
computed field made that dead branch read like a deliberate rule rather than
a copied default.

• agent: Mapped[str], and AgentCall.start(agent: str)
• AgentCallResponse.agent: str; label is get_display_label(self.agent)
• AgentCallSummary.agent: string on the wire mirror
• migration a4e1f60c2b95

The migration adds the constraint with no backfill. A null here would mean a
writer nobody knows about, and inventing an attribution would put it in the
cost table — the constraint should find it instead.

The blank id the constraint would have taken

Agent.id defaults to "" so __set_name__ can fill it, and an Agent
built loose — a local, never assigned to an Extension, never given an
explicit id= — keeps that default. __set_name__ never runs, so it never
registers, but nothing stopped it running: it would have written agent="",
which satisfies NOT NULL and displays as a blank timeline label.

An agent with no id has no durable key — its settings overrides, its registry
entry and its calls all key on nothing — so __call__ refuses it, beside the
check that it is inside a workflow.

Not changing

SubjectStatus.agent stays str | None. Its None means "no live agent call"
— the driving run is parked, or has none yet — not "an unattributed call".