mcp-validate-expression-tool.md

@objectstack/mcp	minor

feat(mcp): validate_expression tool — validate a CEL expression against a schema before authoring (#1928)

Adds an agent-callable MCP tool that runs the same build-time expression checks as objectstack build, so an AI can validate a formula / predicate / flow condition while authoring instead of shipping one that silently evaluates to null. Given { objectName, expression, site? } it resolves the object's real schema (field names + types, via the principal-bound describeObject bridge) and returns:

• errors — bare field refs (amount → record.amount), unknown fields (with a did-you-mean), unknown functions;
• warnings — text/boolean fields misused in arithmetic, date-equality pitfalls;
• inScope — the fields, stdlib functions, and namespace roots available, so the model can self-correct;
• inferredType for a formula site.

site (formula | validation | flow_condition | template, default formula) maps to the validator's role + scope — flow_condition binds fields bare, the rest bind record.<field>. Read-only, gated by the data:read OAuth scope, and fail-closed on sys_* objects like the other schema tools. This is the authoring-time surface the guardrail series (#1928) always pointed at; @objectstack/mcp gains a @objectstack/formula dependency (acyclic; formula is a leaf).