docs(cli): explain set input modes and newline handling

crates/cli/src/main.rs

@@ -181,12 +181,15 @@ enum Command {
         json: bool,
     },
     /// Set a secret. Reads the value from a hidden prompt unless --value/--stdin is given.
+    #[command(
+        after_help = "Input modes:\n  Without --value or --stdin, enter the value at the hidden prompt.\n  --stdin reads until EOF and preserves every byte, including line endings and trailing newlines. It does not trim or interpolate input. A redirected file becomes one complete secret value, not a dotenv import.\n  --value and --stdin cannot be combined. Real credentials passed with --value may appear in shell history and process arguments.\n\nBefore piping input:\n  Initialise the local store and project, and unlock the local identity first. --stdin replaces only the value prompt; it does not bypass these requirements or grant write access through a machine token.\n\nExamples (POSIX shell):\n  sotto set DATABASE_URL\n  sotto set PRIVATE_KEY --stdin < ./example-key.txt\n  printf '%s' 'example-value' | sotto set EXAMPLE --stdin\n\nThe pipeline uses a synthetic value. printf avoids the extra newline commonly added by echo. Do not put real credentials in an inline shell example: they can still enter shell history."
+    )]
     Set {
         name: String,
-        /// Provide the value inline (warning: visible in shell history).
+        /// Provide the value inline (warning: visible in shell history and process arguments).
         #[arg(long, conflicts_with = "stdin")]
         value: Option<String>,
-        /// Read the value from standard input.
+        /// Read the complete value from standard input until EOF, without trimming.
         #[arg(long)]
         stdin: bool,
     },
@@ -2722,6 +2725,32 @@ mod tests {
         assert!(help.contains("Sotto options go before --"));
     }
 
+    #[test]
+    fn set_help_explains_prompt_and_stdin_input() {
+        let mut command = Cli::command();
+        let help = command
+            .find_subcommand_mut("set")
+            .expect("set subcommand should exist")
+            .render_long_help()
+            .to_string();
+
+        for expected in [
+            "sotto set DATABASE_URL",
+            "sotto set PRIVATE_KEY --stdin < ./example-key.txt",
+            "printf '%s' 'example-value' | sotto set EXAMPLE --stdin",
+            "reads until EOF",
+            "trailing newlines",
+            "does not trim or interpolate",
+            "not a dotenv import",
+            "--value and --stdin cannot be combined",
+            "shell history and process arguments",
+            "unlock the local identity first",
+            "synthetic value",
+        ] {
+            assert!(help.contains(expected), "missing {expected:?} in:\n{help}");
+        }
+    }
+
     #[test]
     fn import_help_explains_literal_and_quoted_values() {
         let mut command = Cli::command();