Delivery threads: dedupe and enforce a unique key

[TheGreatAxios]

Summary

createDeliveryThread was select-then-insert on (kind='delivery', runRef) with no unique constraint backing the read — unlike its ensureRootThread/anchoredReplyThread siblings in the same file, fixed for the same bug class in CL-7130. A redelivered routine event could insert a second delivery thread for the same run, splitting replies across both.

• Migration 0026_workbench_threads_delivery_key dedupes any existing duplicate delivery threads (keeping the oldest row per tenant_id, workbench_id, run_ref), repointing workbench_thread_messages.thread_id, workbench_messages.thread_id, and workbench_threads.parent_thread_id off dropped rows before deleting them, then adds a partial unique index scoped to kind = 'delivery' AND run_ref IS NOT NULL.
• createDeliveryThread now inserts with onConflictDoNothing (arbiter predicate matching the index exactly) and re-selects on conflict — the identical structural pattern already used for root/reply threads.

Fixes CL-7199.

Test plan

• cd packages/chat && bun run typecheck — clean
• bun test (full package suite, real Postgres) — 745 pass, 0 fail
• New migration test seeds pre-existing duplicate delivery threads plus null-run-ref rows, a message and a reply thread anchored on a dropped duplicate — asserts dedupe, repoint, null-exclusion, and post-migration index enforcement
• New concurrency test: two racing createDeliveryThread calls for the same run converge on one row
• Verified both new tests fail without the implementation commit (red), pass with it (green)
• bunx prettier --check and bun run lint on touched files — clean

[TheGreatAxios]

critique · reviewed, no blocking findings

Reviewed the two commits (tests, then implementation) against CL-7130's precedent for this exact bug class.

• packages/chat/src/migrations.ts:0026 — repoint-before-delete ordering correct (thread_id refs, then parent_thread_id, then delete duplicates, then index); byte-for-byte structural clone of the proven 0025 migration; null run_ref correctly excluded from dedupe to match the partial index predicate; runs inside the existing per-migration transaction so it's atomic; idempotent via IF NOT EXISTS and the migration ledger.
• packages/chat/src/threads.ts:594-604 — onConflictDoNothing arbiter (target columns + where predicate) matches workbench_threads_delivery_key in schema.ts exactly, same pattern as the root/reply paths.
• Verified independently: reverted the implementation commit and ran the new tests against real Postgres — 3 failures (missing migration, dedupe test, concurrency test diverging on ids). Restored HEAD — 7 pass, 0 fail. tsc clean.
• No signature change to createDeliveryThread; routes.ts's single caller unaffected.

No issues found in scope.