docs(mcp): document Claude Code native HTTP transport

from agent

Document the native claude mcp add --transport http route as the primary Claude Code setup, ahead of the generic mcp-remote stdio wrapper. The native transport handles OAuth in-browser and removes the npx indirection.

Why

Feedback from @WarheadsSE on Orbit Customer Zero, note 3344792122:

I had zero issues operating claude mcp add --transport http gitlab-orbit https://gitlab.com/api/v4/orbit/mcp and then authenticating. Is there some reason we've not declared that route in any of the documentation? Have I missed such?

The mcp-remote wrapper is still the right answer for clients without native streamable-HTTP transport, but Claude Code users were paying for indirection they don't need.

What changed

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

• New Claude Code section with the claude mcp add one-liner.
• Existing JSON config block re-scoped to Cursor, Codex, and other JSON-config clients.
• opencode block unchanged.

Diff

-**Claude Code, Cursor, Codex** — add to your agent's MCP config:
+**Claude Code** supports the Orbit endpoint over the built-in HTTP transport.
+Register it with one command:
+
+\`\`\`shell
+claude mcp add --transport http gitlab-orbit https://gitlab.com/api/v4/orbit/mcp
+\`\`\`
+
+The first \`query_graph\` or \`get_graph_schema\` call opens your browser to
+authenticate with GitLab. No JSON config edit required.
+
+**Cursor, Codex, and other JSON-config clients** — add to your agent's MCP config:

Test plan

• mise lint:docs (markdownlint, Vale, lychee) passes locally.
• CI check-docs job passes.