A workflow's subject is one word, read off the class or off the run

[czpython]

Three things the subject arc left behind.

subject is one word, answering twice

A workflow declares what its runs are about. The declaration and the run's live
subject share the name, so __init_subclass__ moved the class attribute out of
the way and kept a private copy:

del cls.subject
cls._subject_class = declared

Which left Build.subject answering None while Build is plainly about a
WorkItem, and two modules reaching across for the truth — signals.py and
extensions/base.py both read workflow_class._subject_class to derive a
subscriber's filter and an extension's mounted boards.

A descriptor is what the two readings were asking for. The declaration becomes
one, and the same word answers the workflow with the kind of thing and a run
with the particular one:

class _DeclaredSubject:
    def __get__(self, run, owner):
        if run is None:
            return self.subject_class
        ...

Build.subject is WorkItem is true now, _subject_class is gone, and the two
cross-module private reads are workflow.subject.

Presence in cls.__dict__ still decides, not the value: an explicit
subject = None is a declaration of nothing, and stays an error.

The proof extension said "summary" three times

field_notes is what an extension author copies from, and it spelled one word
over three unrelated things: Note.summary (the agent's prose),
Note.get_summary() (the platform's board header), and two classes both named
NoteSummary — one an AgentOutput, one a SubjectSummary.

get_summary() / list_summaries() is the platform's pair and stays. The
domain word moves: a note has a body, and the agent writes its gist.

• Note.summary → Note.gist, save_summary() → save_gist()
• contracts.NoteSummary(AgentOutput) → GistOutput, returning gist
• schemas.NoteSummary(SubjectSummary) keeps its name — it is the platform summary
• the baseline migration, the prompt template and the agent's description follow

Two more builders that were copying fields

Same reduction as #139: a builder that only reads attributes off one object is
model_validate and a few aliases.

FeedItem.from_event — seq / at / kind alias to the event's id /
created_at / type, workflow becomes AliasPath("payload", "kind"), and
id is the f"event:{seq}" computed field DashboardItem.key already models.

AgentCallResponse.from_call — account_username becomes
AliasPath("account", "username") and label a computed field. The two
derivations it reached for move onto the row that owns them:
AgentCall.get_live_status() is the live_status property, and
normalize_token_usage(call.cost_metadata) is AgentCall.token_usage, which
retires the _token_usage helper in schemas.py. status is typed
AgentCallStatus rather than a restated literal union, so the # type: ignore
the mismatch needed goes with it.

Both wire shapes are unchanged, field for field.

The from_* builders that stay are the ones that aren't field copies:
AgentCallFiles.from_call walks the filesystem, RunResponse.from_run takes a
second argument, HarnessResponse.from_row joins three sources,
SettingsFieldResponse.from_field reads a FieldInfo plus kwargs, and the four
from_settings are config constructors.