Self-serve signup & App-install flow

[JSONbored]

Problem: No flow exists for a new customer to sign up, connect their own repository, and grant scoped permissions without engineering involvement.

Area: Product / Frontend

Proposal: Build a self-serve flow: sign up, install the App on the customer's own repository, and confirm the scoped permissions being granted, per the tenancy model in #4783.

Deliverables:

• A working self-serve signup-through-install flow.

Acceptance criteria:

• A new test customer completes signup and install without any manual/engineering step.

Boundaries:

• Depends on the tenancy model established in Multi-tenant hosted architecture spec #4783 (Multi-tenant hosted architecture, closed) and the scoped-credential handling in Scoped credential handling #4794 (closed) — both are done, so this issue is unblocked and ready.

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Part of #4778.

Design system: this signup/install flow is a new UI surface — it must be built on the shared design-system package from #4966/#4967 (the same tokens/components gittensory-ui and the migrated miner-ui both use), not a new one-off styling approach.

[linear-code]

JSO-3474