Fix remote-server docs inconsistency regarding toolset at base url

[korobitsin]

Summary

Clarifies that the base MCP URL (/mcp/) exposes the default toolset rather than all toolsets in the remote server documentation.

Why

The documentation currently implies that the base MCP URL (https://api.githubcopilot.com/mcp/) corresponds to the all toolset. However, the URL path rules later in the document state that / serves the default toolset and /x/all enables all toolsets. This PR resolves that ambiguity.

What changed

• Renamed the toolset entry associated with https://api.githubcopilot.com/mcp/ from all to default.
• Updated the description for that entry from "All available GitHub MCP tools" to "Default toolset".
• This aligns the table with the documented URL path rules where / corresponds to the default toolset and /x/all explicitly enables all toolsets.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• N/A (documentation change only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
• I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (remote-server.md)