docs: add critical warning about silent error trap to error handling guide

[pyramation]

Summary

Adds a prominent "CRITICAL: execute() Does NOT Throw" warning section to the top of codegen-error-handling.md and a brief callout to SKILL.md. This addresses a recurring issue where AI agents wrap .execute() in bare try/catch blocks, silently swallowing errors because .execute() returns a discriminated union and never throws.

Changes (2 files, additive only — no existing content was modified or removed):

• codegen-error-handling.md: New section at top with bug example, fix options (.execute().unwrap() and .ok check), placed before the existing Discriminated Unions section
• SKILL.md: Added error handling callout after Quick Start ORM examples; updated reference table to mark error handling doc as "read first!"

Companion PR: constructive#939 adds the same warning to generated ORM AGENTS.md files via docs-generator.ts.

Review & Testing Checklist for Human

• Verify the warning examples use the correct field names: The new warning section uses result.value and result.error.message to stay consistent with the rest of the existing doc. Confirm these match the actual runtime QueryResult<T> type — if the real type uses result.data / result.errors, both the warning and the rest of the doc need updating together.
• Confirm .execute().unwrap() chaining works at runtime: The warning recommends .execute().unwrap() as the primary fix. Verify this pattern actually works — QueryBuilder.execute() returns Promise<QueryResult<T>>, so .unwrap() must be a method on QueryResult, not just on QueryBuilder.

Notes

• This is documentation-only — no runtime code changes.
• All original content in both files is preserved; changes are purely additive.
• The existing doc's type definitions (result.value, result.error.type with 'graphql' | 'network' | 'validation') were intentionally left as-is — any corrections to those should be a separate PR to avoid scope creep.

Link to Devin session: https://app.devin.ai/sessions/41ed0f7a17024bf19180f5dcade05b76
Requested by: @pyramation

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