docs(mcp): add Gemini CLI native HTTP config and mcp_orbit scope

What does this MR do and why?

Adds a Gemini CLI section to docs/source/remote/access/mcp.md documenting the native HTTP MCP transport config, including the mcp_orbit OAuth scope that Gemini CLI must request explicitly. Without this scope, native HTTP clients fail to authenticate to the Orbit MCP endpoint even when already signed in to GitLab.

The customer's original config in the issue already used httpUrl, which is the documented field for Gemini CLI's streaming HTTP transport (url is reserved for SSE). The actual fix is the missing oauth.scopes: ["mcp_orbit"] block, not a different URL field.

Related Issues

Closes #819 (closed)

Testing

npx markdownlint-cli2 docs/source/remote/access/mcp.md passes with 0 errors.

• This merge request does not introduce any performance regression.

Note on the pipeline: the failed jobs on this fork's pipeline (unit-test, integration-test, corpus-smoke-test, docker-build-mr, and several *-version-check jobs) all fail before their scripts run. The job logs show Pulling docker image gitlab.com:443/ujwal240/dependency_proxy/containers/... returning a 404 ("unexpected end of JSON input"), so the always pull policy fails the job immediately. The CI template resolves the Dependency Proxy prefix to the pipeline's own namespace, and a personal fork namespace doesn't have that proxy populated the way gitlab-org does. unit-test, integration-test, and corpus-smoke-test all pass on main, and this change only touches a markdown file, so these failures are a fork CI/Dependency-Proxy environment issue, unrelated to this MR's content.

Agent context — long-form analysis, file-by-file walkthroughs, profiler output, alternatives considered

Changes to docs/source/remote/access/mcp.md:

• Prerequisites: added a bullet noting native HTTP MCP clients must include the mcp_orbit OAuth scope.
• Connect your MCP client: added a "Gemini CLI" subsection with a working ~/.gemini/settings.json example using httpUrl + oauth.enabled: true + oauth.scopes: ["mcp_orbit"].
• Added a [!note] troubleshooting block: if a native HTTP client fails to authenticate, add oauth.scopes: ["mcp_orbit"] to its MCP server config.

On the httpUrl vs url/type question raised in the issue: per Gemini CLI's documented MCP server schema, httpUrl is the field for streamable HTTP transport and url is reserved for SSE. The customer's failing config in the issue already used httpUrl, so this MR keeps that field and adds only the missing oauth block.