agents: phase tracking, pid files, pipelining, unified cycle

- AgentStep with phase labels (=== PROMPT phase:name ===)
- PID files in state dir (pid-{PID} with JSON phase/timestamp)
- Built-in bail check: between steps, bail if other pid files exist
- surface_observe_cycle replaces surface_agent_cycle + journal_agent_cycle
- Reads surface output from state dir instead of parsing stdout
- Pipelining: starts new agent if running one is past surface phase
- link_set upserts (creates link if missing)
- Better error message for context window overflow

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

src/cli/agent.rs

@@ -92,11 +92,11 @@ pub fn cmd_consolidate_batch(count: usize, auto: bool, agent: Option<String>) ->
 
     if let Some(agent_name) = agent {
         let batch = crate::agents::prompts::agent_prompt(&store, &agent_name, count)?;
-        for (i, p) in batch.prompts.iter().enumerate() {
-            if batch.prompts.len() > 1 {
-                println!("=== STEP {} ===\n", i + 1);
+        for (i, s) in batch.steps.iter().enumerate() {
+            if batch.steps.len() > 1 {
+                println!("=== STEP {} ({}) ===\n", i + 1, s.phase);
             }
-            println!("{}", p);
+            println!("{}", s.prompt);
         }
         Ok(())
     } else {