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/function/mod.rs

@@ -517,7 +517,11 @@ impl Functions {
                 anyhow!("Failed to load embedded function file: {}", file.as_ref())
             })?;
             let content = unsafe { std::str::from_utf8_unchecked(&embedded_file.data) };
-            let file_path = paths::functions_dir().join(file.as_ref());
+            let file_path = if file.as_ref() == "mcp.json" {
+                paths::mcp_config_file()
+            } else {
+                paths::functions_dir().join(file.as_ref())
+            };
             let is_script = file_path
                 .extension()
                 .and_then(OsStr::to_str)