docs(client): drop the retired validateOnly batch option from the README (#4443)

The Batch Options section documented `validateOnly` as a working dry-run —
"validate records without persisting changes" — but the key was retired in
#4052 precisely because nothing ever read it. Every batch surface
(updateManyData / deleteManyData / batchData) persisted regardless, so a
caller who sent it to preview a mutation got that mutation executed.

BatchOptionsSchema has carried a retiredKey(...) tombstone since #4052, so
the schema already refuses the key loudly. The README was the last place
still promising it — declared-but-not-enforced in prose rather than in code,
aimed at exactly the readers who cannot see the tombstone.

Replaced with a pointer to docs/protocol-upgrade-guide.md
(batch-options-validate-only-retired). No behaviour change; there is no batch
dry-run today. Shipped as a patch because README.md is in this package's
`files`, so the correction only reaches npmjs.com readers if it releases.

Found while evaluating #4372 (write-path validate-only mode), closed as not
planned — no current consumer justifies the surface.

Author: os-zhuang

.changeset/client-readme-retired-validate-only.md

@@ -0,0 +1,25 @@
+---
+'@objectstack/client': patch
+---
+
+docs(client): drop the retired `validateOnly` batch option from the README (#4052)
+
+The Batch Options section still documented `validateOnly` as a working dry-run —
+"validate records without persisting changes" — but the key was retired in #4052
+precisely because nothing ever read it. Every batch surface (`updateManyData` /
+`deleteManyData` / `batchData`) persisted regardless, so a caller who sent it to
+preview a mutation got that mutation **executed**.
+
+`BatchOptionsSchema` has carried a `retiredKey(...)` tombstone since #4052, so the
+schema already refuses the key loudly. The README was the last place still
+promising it — declared-but-not-enforced in prose rather than in code, aimed at
+exactly the readers who cannot see the tombstone.
+
+Released as a patch rather than declared release-nothing because `README.md` is in
+this package's `files`: the corrected text only reaches the people who hit the
+problem — readers on npmjs.com — if the package ships.
+
+Replaced with a pointer to `docs/protocol-upgrade-guide.md`
+(`batch-options-validate-only-retired`). No behaviour change; there is no batch
+dry-run today. Write-path validate-only was evaluated in #4372 and closed as not
+planned — no current consumer justifies the surface.

packages/client/README.md

@@ -134,7 +134,10 @@ Batch operations support the following options:
 - `atomic`: If true, rollback entire batch on any failure (default: true).
 - `returnRecords`: If true, return full record data in response (default: false).
 - `continueOnError`: If true (and atomic=false), continue processing remaining records after errors.
-- `validateOnly`: If true, validate records without persisting changes (dry-run mode).
+
+> `validateOnly` was retired in #4052 — it was never implemented and batch surfaces persisted
+> regardless, so there is no batch dry-run today. Drop the key; see
+> `docs/protocol-upgrade-guide.md` (`batch-options-validate-only-retired`).
 
 ### Error Handling
 The client provides standardized error handling with machine-readable error codes: