V2: Add service QR pairing and connection-details endpoint #35943

Summary

Add a first-class V2 pairing flow for connecting OpenCode clients such as OpenMobile to an existing service:

• opencode2 service qr renders a scannable QR code for the current service.
• An authenticated server endpoint returns the canonical connection/pairing details so already-connected clients (including the TUI) can render the same QR or deep link.

This replaces helper scripts that discover service details and construct QR payloads outside OpenCode.

Proposed behavior

• The CLI obtains all connection details through the service/CLI APIs rather than reading or manipulating OpenCode state files directly.
• The QR encodes a versioned OpenMobile-compatible deep link that can add/open the server.
• The server exposes a typed, documented response containing the canonical pairing payload and structured connection details needed by clients.
• Connected clients can use the endpoint to show a QR without reconstructing service configuration independently.
• Authentication secrets are returned only to an already-authenticated caller and must not appear in logs or diagnostics.
• The schema can represent the currently reachable address (including Tailscale) and later accommodate OpenTunnel without changing client-specific parsing logic.

Deliverables

• opencode2 service qr
• Authenticated server endpoint and OpenAPI schema
• Shared/versioned deep-link or pairing schema
• Tests for CLI rendering, endpoint authorization, and payload round-tripping
• Send the finalized schema to the OpenMobile client owner for integration

Open questions

• Should the endpoint return only structured data/deep-link text, or also a rendered QR representation?
• How should a service select or advertise among localhost, LAN, Tailscale, and future OpenTunnel addresses?
• Should pairing credentials be reusable service credentials or short-lived/revocable tokens?

Source: https://slack.com/archives/C0HJ2SVJL/p1783531107717499