Give workflow persistence a dedicated repository

[boudra]

PostgreSQL workflow persistence now has a dedicated repository, so workflow changes no longer require navigating the broad PgDatabase implementation. The existing Database contract and runtime behavior remain unchanged.

Goals

• Give durable workflow SQL, transaction recovery, row mapping, and activity reads one explicit owner.
• Preserve the existing public database interface and transaction semantics.
• Keep shared agent-execution insertion, metering, and non-workflow transitions behind narrow injected operations.
• Remove workflow row types and workflow persistence helpers from PgDatabase.

Non-goals

• No workflow product or behavior changes.
• No database schema or migration changes.
• No refactor of the in-memory database implementation.
• No changes to daemon lifecycle behavior.

Why

src/db/pg.ts had grown to nearly 5,000 lines, with durable workflow persistence occupying its largest coherent responsibility cluster. Existing connection and provider-event repositories already establish a repository boundary for PostgreSQL domains; this applies the same ownership model to workflows while leaving the stable Database interface intact.

Verification

• npm test — 643 passed, 14 skipped
• npm run typecheck
• npm run lint:node
• npm run format:check
• npm run db:check

Risk surface

The meaningful risk is transaction behavior at the workflow/agent-execution boundary: atomic execution creation and usage reservation, deadline recovery, terminal notification leasing, and idle-deadline synchronization. The PostgreSQL workflow integration suite and full test suite cover these paths. There are no schema or externally observable changes.

Follow-up candidates

• Decompose the large Hub E2E helper by test-domain ownership.
• Separate daemon lifecycle persistence and orchestration responsibilities.