Neuron/hotkey registration flow

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Problem: No flow exists for registering a customer's rented compute-time allocation on the gittensor subnet.

Area: gittensor economics

Proposal: Implement the registration mechanism defined by #4781, allocating compute-time to a specific customer/loop without exposing any subnet credential material in application logs, issue text, or client-visible output.

Deliverables:

• A working registration flow tying a rented loop to its allocated compute-time on the subnet.

Acceptance criteria:

• A test registration completes and is verifiable without any credential material appearing in logs or output.

Boundaries:

• Blocked on gittensor rental economics spec #4781's sign-off.
• No wallet addresses, hotkeys, or other credential material in code comments, commit messages, or issue/PR text — handle exclusively through secret storage.

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Part of #4778.

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JSO-3461