docs(acp): add session sync spec

[zerob13]

Summary

• Add SDD documents for ACP session synchronization.
• Document session/list import, load/resume reuse, stale remote handling, and ACP-backed local removal semantics.
• Clarify that this PR only contains the design/spec plan and does not implement the feature yet.

Implementation Status

• Spec only.
• No runtime code changes.
• No UI changes.
• No tests added yet.

Validation

• Commit hook ran typecheck:node and typecheck:web successfully.

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