api: retry transient connection errors, misc fixes

- Retry up to 5 times with exponential backoff (2s, 4s, 8s, 16s)
  on transient errors: IncompleteMessage, connection closed/reset/
  refused, timeouts. Non-transient errors fail immediately.
- tail command: print to stdout instead of stderr
- state_dir rename: output_dir → state_dir throughout knowledge.rs

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/cli/journal.rs

@@ -33,18 +33,18 @@ pub fn cmd_tail(n: usize, full: bool) -> Result<(), String> {
         };
         let del = if node.deleted { " [DELETED]" } else { "" };
         if full {
-            eprintln!("--- {} (v{}) {} via {} w={:.3}{} ---",
+            println!("--- {} (v{}) {} via {} w={:.3}{} ---",
                 node.key, node.version, ts, node.provenance, node.weight, del);
-            eprintln!("{}\n", node.content);
+            println!("{}\n", node.content);
         } else {
             let preview = crate::util::first_n_chars(&node.content, 100).replace('\n', "\\n");
-            eprintln!("  {}  v{}  w={:.2}{}",
+            println!("  {}  v{}  w={:.2}{}",
                 ts, node.version, node.weight, del);
-            eprintln!("  {}  via {}", node.key, node.provenance);
+            println!("  {}  via {}", node.key, node.provenance);
             if !preview.is_empty() {
-                eprintln!("  {}", preview);
+                println!("  {}", preview);
             }
-            eprintln!();
+            println!();
         }
     }