Schema: Add ClickToSign to component.schema.json

[mikewolfd]

Motivation

The component JSON schema needs a ClickToSign definition so that component documents using ClickToSign validate correctly.

Scope

• Add $defs/ClickToSign to schemas/component.schema.json following the pattern of other Progressive input components (e.g., Signature, Rating)
• Include: component const, bind (required), statement, buttonLabel, showTimestamp, showSignerName, requireTypedName, revocable props
• Set x-lm metadata: category input, level progressive, children false, bind required, fallback Stack
• Add { "$ref": "#/$defs/ClickToSign" } to the AnyComponent oneOf union
• Add "ClickToSign" to the built-in component name exclusion list in CustomComponentRef
• Add example entry
• Run npm run docs:generate and npm run docs:check

Acceptance Criteria

• ClickToSign component nodes validate against the schema
• Invalid ClickToSign nodes (missing bind, unknown props) are rejected
• Schema artifacts regenerated and checks passing

Dependencies

Depends on #2 (spec definition should be finalized before encoding in schema).