Add quiet flag to list command for machine-readable output

- Add ListArgs struct with quiet boolean flag
- Convert List command from unit variant to struct variant
- In quiet mode: no headers, colon-separated format (id:pid:status)
- In normal mode: preserve existing table format
- Handle quiet mode in all output scenarios (normal, error cases)
- Add comprehensive test for quiet mode functionality

Example usage:
  demon list      # Normal table output
  demon list -q   # Quiet: process1:123:RUNNING

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

1 files changed, 33 insertions, 0 deletions

diff --git a/tests/cli.rs b/tests/cli.rs
index 7cb68b7..05e3efd 100644
--- a/tests/cli.rs
+++ b/tests/cli.rs
@@ -343,4 +343,37 @@ fn test_invalid_process_id() {
         .assert()
         .success()
         .stdout(predicate::str::contains("invalid PID file"));
+}
+
+#[test]
+fn test_list_quiet_mode() {
+    let temp_dir = TempDir::new().unwrap();
+    
+    // Test quiet mode with no processes
+    let mut cmd = Command::cargo_bin("demon").unwrap();
+    cmd.current_dir(temp_dir.path())
+        .args(&["list", "--quiet"])
+        .assert()
+        .success()
+        .stdout(predicate::str::is_empty());
+    
+    // Create a process
+    let mut cmd = Command::cargo_bin("demon").unwrap();
+    cmd.current_dir(temp_dir.path())
+        .args(&["run", "--id", "quiet-test", "echo", "done"])
+        .assert()
+        .success();
+    
+    // Test quiet mode with process - should output colon-separated format
+    let mut cmd = Command::cargo_bin("demon").unwrap();
+    cmd.current_dir(temp_dir.path())
+        .args(&["list", "-q"])
+        .assert()
+        .success()
+        .stdout(predicate::str::contains("quiet-test:"))
+        .stdout(predicate::str::contains(":DEAD"))
+        // Should not contain headers
+        .stdout(predicate::str::contains("ID").not())
+        .stdout(predicate::str::contains("PID").not())
+        .stdout(predicate::str::contains("STATUS").not());
 }
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