docs(target-arch): revise coordination model to pull/work-stealing (#1081)

[vybe]

Summary

Revises docs/planning/TARGET_ARCHITECTURE.md §Coordination Model from a push actor model (backend dispatches into a per-agent Redis mailbox) to pull / work-stealing, per the 2026-06-05 design review. The backend owns one durable per-agent queue (schedule_executions); each agent pulls the next task only when it has a free worker; capacity is physical (worker count); the sole recovery primitive is lease-expiry re-delivery of the same execution_id.

Chosen over the push model for simplicity, single-source-of-truth, and ~80% reuse of existing primitives (claim_next_queued, the RELIABILITY-005 CAS, idempotency #525, the agent auto_sync.py loop).

What changed in the doc

• Governing principle Setup improvements #5 → pull-based work-stealing.
• Data layer — queue + execution state move to Postgres (one owner of "queued" + "running"); Redis loses the mailbox STREAM and slot ZSET, keeps only a wake-up hint.
• Coordination Model rewritten: backend queue, pull endpoints, physical worker pool, the two operator levers, lease-expiry re-delivery, fan-out join, failure isolation without a dispatch breaker, replica groups via row-claim.
• New side-effect idempotency contract — the hardest open problem (feat: effect-scoped idempotency keys for outbound side effects #1084): re-delivery is safe for bookkeeping but re-emits external effects; effect-scoped keys gate pull-on for side-effect agents.
• Agent runtime / observability — pull worker pool; mailbox_depth dropped from /health; journal demoted from source-of-truth; queue-depth metrics.
• Open questions + tracking table reconciled with the new issues.

Tracking (Epic #1045)

Issue	Role
#1081	umbrella — 6-phase pull migration
#1082 / #1083	bankable wins (status-as-projection / fire-and-forget) — ship independently
#1084	effect-scoped idempotency — the gate
#1085	correlated-failure / herd controls
#946 / #429 / #300	pilot / cleanup payoff / Postgres prerequisite
#307 / #526	repurposed gate→alert under pull

Also includes the team announcement record under docs/user-docs/dev-announcements/.

Docs-only; no code changes.

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