feat: prefer ~/.config/coyote/mcp.json for the user-scope MCP config

The user-scope MCP config historically lived at
<config-dir>/functions/mcp.json, a leftover from when MCP support was
part of the llm-functions tooling. It now resolves through a single
choke point with these semantics:

- Preferred location: <config-dir>/mcp.json (created there on first run)
- Historical <config-dir>/functions/mcp.json still honored when the
  preferred file does not exist, so existing installs are unchanged
- If both exist, the preferred location wins

--info/.info now reports the resolved location as mcp_config_file, and
the --scope help text plus config.agent.example.yaml reference the new
default. This also removes the asymmetry with the workspace scope,
which already used .coyote/mcp.json directly.

src/config/request_context.rs

@@ -1865,6 +1865,7 @@ impl RequestContext {
             ("rags_dir", display_path(&paths::rags_dir())),
             ("macros_dir", display_path(&paths::macros_dir())),
             ("functions_dir", display_path(&paths::functions_dir())),
+            ("mcp_config_file", display_path(&paths::mcp_config_file())),
             ("sbx_kit_dir", display_path(&paths::sbx_kit_dir())),
             ("messages_file", display_path(&self.messages_file())),
         ];