Validation `events: ['delete']` is a silent no-op — trim it from the spec

[os-zhuang]

Summary

BaseValidationSchema admits events: ['insert', 'update', 'delete'] (packages/spec/src/data/validation.zod.ts:88), but the rule evaluator's runtime Mode is 'insert' | 'update' (packages/objectql/src/validation/rule-validator.ts:72) and engine.delete never invokes evaluateValidationRules. The events filter (rule-validator.ts:526) can therefore never match 'delete' — a validation rule declaring events: ['delete'] silently never runs.

This is the declared ≠ enforced tail split out of #3106 / PR #3160 (which fixed the multi-row-update call-site gap and left this one explicitly for a follow-up). It was also independently flagged by the June liveness audit.

Evidence

• packages/spec/src/data/validation.zod.ts:88 — events: z.array(z.enum(['insert', 'update', 'delete'])).default(['insert', 'update']).
• packages/objectql/src/validation/rule-validator.ts:72 — type Mode = 'insert' | 'update'; :526 — const events = r.events ?? ['insert', 'update']; return events.includes(mode); (mode is never 'delete').
• docs/audits/2026-06-validationschema-property-liveness.md:10-21 — "🔴 events: ['delete'] is a silent no-op … Remove delete from the events enum (dead)."
• The published skill skills/objectstack-data/rules/validation.md:268 actively teaches events: ['delete'] ("Only on delete"), so AI-authored metadata following the skill gets a delete guard that never fires.

Blast radius

Zero in practice. No example app, seed, template, fixture, or test declares a validation rule with 'delete' in events — the only repo-wide occurrences are documentation/skill text. Any hypothetical off-spec metadata fails loudly (ZodError at registry.validate / os validate), never silently.

Decision: trim (not enforce)

Remove 'delete' from the events enum (contract-first, AGENTS.md PD #12). Delete-time guards belong in beforeDelete lifecycle hooks, which the skills already teach correctly (skills/objectstack-data/references/data-hooks.md:448-464). Enforcement (wiring the evaluator into engine.delete) was considered and rejected: only script/cross_field would be meaningful on delete, and hooks already cover that need.

Scope

• Narrow the enum + its three hand-written mirrors (validation.zod.ts:226 type, rule-validator.ts:79 BaseRule, packages/metadata-protocol/src/protocol.ts:231-235 JSON-schema mirror).
• Fix the skill line and the hand-written doc table (content/docs/data-modeling/validation.mdx:72); regenerate content/docs/references/data/validation.mdx.
• Correct the doc comments that framed the delete gap as a known exclusion.
• Changeset + a negative test asserting events: ['delete'] is now rejected.

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