feat(cli): manage synced sidebar sections

[marcusquinn]

Summary

• add buzz sidebar sections list/apply plus create, assign, unassign, rename, reorder, and delete operations for the CLI's current identity
• reuse Desktop's encrypted kind 30078, d=channel-sections wire payload and NIP-44 self-encryption
• resolve exact channel/section names to stable IDs and reject missing or ambiguous references
• require revision compare-and-set checks, recheck before publish, and verify the resulting remote head
• keep full-layout reapplication idempotent and reject malformed, stale, oversized, or orphaned layouts

Scope

This implements the direct current-identity CLI portion of Ref #3590. It does not let a managed agent sign or encrypt as its owner. The owner-reviewed Desktop broker remains separate work in #5353 and #5352.

Verification

• cargo test -p buzz-cli --no-fail-fast (353 passed)
• cargo clippy -p buzz-cli --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

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