docs: Spec and plan awaiting signals

[themusicman]

Summary

• Spec for Oban-style mid-handler await_signal / signal on PgFlow Jobs and Flows
• Implementation plan (helpers V05, Queries.Signals, Context/Client API, worker park catch, timeout sweeper, Job/Flow coverage)
• Design choices locked: run_id + step_slug addressing, full buffer/timeout parity, re-enter handler from top, Elixir-owned park + task_signals store

Detailed changes

Spec (docs/superpowers/specs/2026-08-21-await-signals-design.md)

• Park tasks as waiting, free worker slot, resume via PgFlow.signal/3,4
• Early signal buffering (last write wins), live wait_timeout, persisted wait_for
• JSONB payloads only in v1; no cancel API or dashboard polish

Plan (docs/superpowers/plans/2026-08-21-await-signals.md)

• Six TDD tasks with concrete files, SQL interfaces, and test commands
• Clarifies production catch lives in Worker.Server (not unused Executor path)

Test plan

• Review spec for API/lifecycle correctness vs Oban Pro awaiting signals
• Review plan file map and Task 1 SQL against current helpers migration pattern
• Confirm unrelated untracked conditional-step docs/migrations are not included
• After merge (or on implementation PR): execute plan tasks and run listed mix test targets

Notes

Documentation-only PR — no runtime code yet. Implementation should follow the plan on this branch or a follow-up.