Revamp CloudSaveKit integration documentation

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Summary

• update the package quick start to CloudSaveKit 0.2.4 and make required application capabilities explicit
• add public DocC guides for integration, explicit synchronization, private-zone sharing, and troubleshooting
• document signing verification, checkpoint provenance, operation-count semantics, and two-device acceptance without consumer-specific implementation details

Validation

• AgentGuidelines consumer validation passes at 0.0.32
• Markdown wrapping check passes
• static-hosting DocC archive builds without warnings
• git diff check passes
• repository ignore policy matches the shared template

Scope

Documentation only. No public API, runtime, dependency, or test changes.

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ChatGPT exact-head documentation review: APPROVED

• Reviewed base: a277700
• Reviewed head: 8c79671
• P0/P1 findings: None
• Prior P1 resolved: deletion acknowledgements and account-transition persistence responsibilities now match the public API lifecycle
• Exact-head CI: Swift Format and Test both pass
• Local DocC static-hosting archive generation completed without warnings