Use ULID instead of UUID for run IDs

ULIDs are lexicographically sortable by creation time, making log
directories and run lists naturally ordered without extra metadata.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -14,6 +14,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["full"] }
 reqwest = { version = "0.12", default-features = false, features = ["json", "stream", "rustls-tls"] }
+ulid = "1"
 uuid = { version = "1", features = ["v4"] }
 rand = "0.8"
 dotenvy = "0.15"

crates/arc-attractor/Cargo.toml

@@ -32,6 +32,7 @@ thiserror.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 tokio.workspace = true
+ulid.workspace = true
 uuid.workspace = true
 rand.workspace = true
 async-trait.workspace = true

crates/arc-attractor/src/cli/run.rs

@@ -480,7 +480,7 @@ pub async fn run_command(args: RunArgs, styles: &'static Styles) -> anyhow::Resu
     let engine = PipelineEngine::with_interviewer(registry, Arc::clone(&emitter), interviewer, Arc::clone(&execution_env));
 
     // 7. Execute
-    let run_id = worktree_run_id.unwrap_or_else(|| uuid::Uuid::new_v4().to_string());
+    let run_id = worktree_run_id.unwrap_or_else(|| ulid::Ulid::new().to_string());
     let config = RunConfig {
         logs_root: logs_dir.clone(),
         cancel_token: None,
@@ -602,7 +602,7 @@ fn setup_worktree(
 ) -> anyhow::Result<(String, PathBuf, PathBuf, String, String)> {
     let base_sha = crate::git::head_sha(original_cwd)
         .map_err(|e| anyhow::anyhow!("{e}"))?;
-    let run_id = uuid::Uuid::new_v4().to_string();
+    let run_id = ulid::Ulid::new().to_string();
     let branch_name = format!("arc/run/{run_id}");
     crate::git::create_branch(original_cwd, &branch_name)
         .map_err(|e| anyhow::anyhow!("{e}"))?;

crates/arc-attractor/src/server.rs

@@ -170,7 +170,7 @@ async fn start_pipeline(
         }
     };
 
-    let run_id = uuid::Uuid::new_v4().to_string();
+    let run_id = ulid::Ulid::new().to_string();
     let interviewer = Arc::new(WebInterviewer::new());
     let (event_tx, _) = broadcast::channel(256);
     let (cancel_tx, cancel_rx) = tokio::sync::oneshot::channel::<()>();