fix: retry branch reconciliation without re-executing effects

[chubes4]

Summary

• persist terminal reconcile receipts in each branch's existing durable descriptor row before reconciliation
• retry lock contention through a dedicated reconcile-only Action Scheduler hook that never calls branch execution
• reconcile missing or expired descriptors through deterministic receipt-only rows
• preserve custom branch-store ownership and carry opaque continuation state for multi-phase reconciliation
• include reconcile retries in scoped drains and run-scoped terminal cleanup

Closes #533.

Root cause

reconcile_branch_result() retried agents_reconcile_lock_unavailable by enqueueing the original branch hook and payload. The new action entered run_branch_action(), rehydrated the descriptor, and executed completed branch steps again before making another reconciliation attempt.

Behavior change

A completed branch now stores a read-after-write-verified reconcile receipt before the first reconciliation attempt. The receipt normally lives inside the branch's already-indexed descriptor row, avoiding concurrent appends to the shared run index. A failed write stops the original action before reconciliation rather than enqueueing an unreadable retry.

When a built-in descriptor is missing or expired, its deterministic store_ref receives a verified receipt-only row instead. No shared-index write occurs. The reconcile-only retry records the terminal workflow_branch_descriptor_missing result, then deletes that exact row after successful reconciliation. Expired local rows also delete themselves when read.

Lock contention enqueues RECONCILE_HOOK with a small durable receipt reference. Its callback reloads that receipt and calls only the idempotent reconciliation path; it cannot re-enter execute_branch(). Late duplicate retries whose receipts were already cleaned up verify the authoritative completed state and no-op.

Consumer-owned refs continue through the existing wp_agent_workflow_branch_store_put, wp_agent_workflow_branch_store_get, and wp_agent_workflow_branch_store_forget contract. Missing custom descriptors create receipt-only records only through those owner filters, and exact receipt cleanup also stays in that store. No local option fallback occurs.

Receipts persist opaque reconcile_continuation error data and expose wp_agent_workflow_reconcile_retry, allowing PR #536's authoritative multi-phase continuation to advance after rebase instead of relying only on completed-handle redelivery.

Regression coverage

• side-effect counter remains exactly 1 through lock contention and duplicate reconcile delivery
• failed receipt write plus forced lock contention performs no reconcile and queues no stranded retry
• stale concurrent shared-index state cannot orphan terminal receipt options during cleanup
• missing and expired descriptors survive contention, retry reconciliation, and reach terminal workflow failure
• receipt-only built-in rows and custom-store receipts are removed after successful reconciliation
• custom stores own receipt persistence, retrieval, and cleanup without local fallback
• opaque authoritative continuation state survives into the reconcile-only retry seam

Tests

• php tests/workflow-as-branch-smoke.php
• php tests/workflow-async-branch-payload-smoke.php
• php tests/workflow-request-controller-smoke.php
• php tests/workflow-scoped-drain-smoke.php
• composer test
• composer phpstan
• git diff --check

AI assistance

Implemented with OpenCode AI assistance. The implementation, review remediation, diff, and test results were reviewed during development.