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/cli/mod.rs

@@ -315,7 +315,7 @@ pub struct Cli {
     /// URL for http/sse MCP server (used with --mcp-add)
     #[arg(long, value_name = "URL", help_heading = "MCP Servers")]
     pub url: Option<String>,
-    /// Scope for MCP config: user (~/.config/coyote/functions/mcp.json) or workspace (./.coyote/mcp.json). Default: user
+    /// Scope for MCP config: user (~/.config/coyote/mcp.json) or workspace (./.coyote/mcp.json). Default: user
     #[arg(long, value_enum, value_name = "SCOPE", help_heading = "MCP Servers")]
     pub scope: Option<McpScopeArg>,
     /// Environment variable for stdio MCP server (repeatable): --env KEY=VALUE