D1030 — Antigravity emits canonical "url" key for MCP Streamable HTTP

Decision. emitAntigravityServer in internal/configurator/configurator.go emits "url": spec.URL instead of "serverUrl": spec.URL for HTTP server entries in .gemini/config/mcp_config.json (and .agents/mcp_config.json).

This amends D194 §MCP Configuration Emission, which emitted "serverUrl" for Antigravity.

Why. When Antigravity support was initially introduced in D194, Antigravity/Gemini CLI used "serverUrl" for remote HTTP server entries. With the release of Antigravity's Streamable HTTP transport support (aligned with the MCP Streamable HTTP 2025-03-26 specification), Antigravity promoted "url" to the canonical and required key (https://antigravity.google/docs/mcp), maintaining "serverUrl" solely as a backwards-compatible fallback.

Furthermore, across all other six providers supported by Cartographer (claude, codex, copilot, crush, kiro, opencode), the emitters already generate "url". Aligning Antigravity with "url" ensures consistency across all provider emitters in Cartographer and adopts Antigravity's preferred modern schema.

Alternatives rejected. - Emit both "url" and "serverUrl". Antigravity prioritizes "url" and documents it as the required key; duplicate keys in user JSON configuration files are redundant and non-standard. - Keep "serverUrl". Retaining legacy keys when the upstream client has modernized creates divergence and risks deprecation breakage in future Antigravity releases.

Consequences. - cartographer connect antigravity and cartographer sync emit "url" in mcp_config.json. - Non-destructive mergeJSON continues preserving pre-existing user configurations. - An entry written before this decision is upgraded in place: when the emitted entry carries "url", mergeJSON drops a "serverUrl" left in that same entry, so the old address does not linger beside the new one. Other servers in the file are untouched.