Subject keying rides DBOS workflow attributes; durable_runs shrinks

[czpython]

What

Runs are keyed to their subject through DBOS custom workflow attributes instead of druks-owned columns.

• Workflow.start() wraps the enqueue in SetWorkflowAttributes({subject_type, subject_id, extension}) — subject_id always stamped as a string, the one shape every reader compares. Scheduled framework crons stay attribute-less (subjectless by design).
• The platform's subject queries (Run.list_for_subject, AgentCall.list_for_subject) filter through one internal subject_filter EXISTS predicate on workflow_status.attributes. AgentCall.list_for_subject no longer touches durable_runs at all. The predicate takes a fresh alias per call, so it stays independent of the state/updated_at scalar subqueries (which claim the bare table via correlate_except).
• Extensions stay on the author door: Run.list_for_subject grows a kind filter, and build's newest-scope lookup and Scope.parked_for compose it — no engine-internal imports in extension code.
• durable_runs shrinks: subject, extension, and input columns are gone. The row keeps only the facts DBOS has no slot for — the gate ask triple, failure/failure_code, kind, created_at. (input had zero readers; DBOS keeps its own copy of workflow arguments for recovery.)
• Events and signals take subject/extension from the workflow's own arguments — replay-deterministic, and payload id typing is unchanged for subscribers. The park notification receives the subject from the parking workflow; the transcript-files route names its own mount instead of reading the run's stamp.

No wire changes: RunResponse never carried subject, extension, or input.

Migration

Drops the three columns, nothing else — every host already runs dbos 2.26, so workflow_status.attributes exists. Runs started before this deploy carry no attributes and drop out of subject timelines; new runs key themselves at start().

Operational note: DBOS workflow_status GC/retention must stay off — a purged workflow row now also unkeys its run's timeline, not just its derived state.

Tests

• seed_dbos_status stamps attributes (the shape start() writes); every seeding suite moved its subject/extension keying there.
• New pin in test_durable_sdk: a subjected start through the live DBOS enqueue lands {"subject_type", "subject_id"} on workflow_status.attributes, id normalized to string.
• The Run.input write-shape pin is replaced by asserting the input round-trips into the body through DBOS's checkpointed arguments.

🤖 Generated with Claude Code

[czpython]

Folded into #4, which now targets main and carries this branch's commits verbatim (subject keying via DBOS attributes) plus the extension-as-class-identity work that removes the per-run extension entirely. Review history stays here; nothing from this branch lands separately.