feat: M3.7.7 signature extraction CLI + integration tests (unit tests pass, integration tests pending mem-cli fix)

crates/mem-cli/src/main.rs

@@ -136,6 +136,16 @@ enum Commands {
         #[arg(long)]
         database_url: Option<String>,
     },
+
+    /// Extract and explain failure signature
+    Sig {
+        /// Tool name (e.g. npm, cargo, kubectl)
+        #[arg(long, value_name = "TOOL")]
+        tool: String,
+        /// Read failure log from file (stdin if not specified)
+        #[arg(long, value_name = "FILE")]
+        file: Option<PathBuf>,
+    },
 }
 
 #[tokio::main]
@@ -191,6 +201,9 @@ async fn main() -> anyhow::Result<()> {
             };
             cmd_verify(&project, db, log, log_dir, output_format, &database_url).await?
         }
+        Commands::Sig { tool, file } => {
+            cmd_sig(&tool, file.as_ref())?
+        }
     }
 
     Ok(())
@@ -356,3 +369,36 @@ async fn cmd_verify(
 
     Ok(())
 }
+
+fn cmd_sig(tool: &str, file: Option<&PathBuf>) -> anyhow::Result<()> {
+    use mem_core::lesson;
+    use std::io::Read;
+
+    // Read failure log from file or stdin
+    let mut output = String::new();
+    if let Some(file_path) = file {
+        output = fs::read_to_string(file_path)?;
+    } else {
+        std::io::stdin().read_to_string(&mut output)?;
+    }
+
+    // Extract signature
+    match lesson::extract(tool, &output) {
+        Some(sig) => {
+            println!("=== Failure Signature ===");
+            println!("Tool: {}", sig.tool);
+            println!("Rule: {}", sig.rule);
+            println!("Hash (SHA256): {}", sig.sig_sha);
+            println!("\n=== Raw Error ===");
+            println!("{}", sig.raw);
+            println!("\n=== Normalised Form ===");
+            println!("{}", sig.normalised);
+        }
+        None => {
+            eprintln!("Failed to extract signature for tool: {}", tool);
+            std::process::exit(1);
+        }
+    }
+
+    Ok(())
+}