refactor: update schema export docs from flat props to nested schema object

[pyramation]

Summary

Updates all skill documentation to reflect the schema export config refactor from flat properties to a nested object, matching constructive-io/constructive#848.

Before: schemaOnly: true, schemaOnlyOutput: './schemas', schemaOnlyFilename: 'public.graphql'
After: schema: { enabled: true, output: './schemas', filename: 'public.graphql' }

Changes across 3 docs files + rebuilt zip:

• SKILL.md — Updated code examples in Schema Export section and GenerateOptions interface definition
• references/generate-schemas.md — Updated all code examples, options table, generateMulti() usage, and behavioral description
• references/cli-reference.md — Added new Schema Export Options table (--schema-enabled, --schema-output, --schema-filename)

Review & Testing Checklist for Human

• Verify behavioral description accuracy: generate-schemas.md line 61 now states schema export works alongside generators when both are enabled. Confirm this matches the actual implementation in constructive#848 — the old behavior was mutually exclusive (schema export OR code generation, never both).
• Verify CLI flags match implementation: The new CLI flags --schema-enabled, --schema-output, --schema-filename in cli-reference.md should match what was implemented in constructive's cli/index.ts and cli/handler.ts.
• Ensure constructive#848 merges first: These docs will be incorrect until the companion implementation PR lands. Consider merge ordering.

Notes

• The .zip file shows as newly created because it wasn't previously tracked. It's the standard skill packaging format per AGENTS.md.
• constructive-private-skills and constructive-db were audited and have no references to the old schema config syntax (no changes needed).

Link to Devin session: https://app.devin.ai/sessions/a0ac766d75d340d490fd265bfafaac83
Requested by: @pyramation

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