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/subconscious/llm.rs

@@ -22,16 +22,18 @@ pub(crate) fn call_simple(caller: &str, prompt: &str) -> Result<String, String>
     };
 
     let prompts = vec![prompt.to_string()];
-    super::api::call_api_with_tools_sync(caller, &prompts, None, &log)
+    super::api::call_api_with_tools_sync(caller, &prompts, None, None, &log)
 }
 
 /// Call a model using an agent definition's configuration (multi-step).
+/// Optional bail_fn is called between steps — return Err to stop the pipeline.
 pub(crate) fn call_for_def_multi(
     def: &super::defs::AgentDef,
     prompts: &[String],
+    bail_fn: Option<&(dyn Fn(usize) -> Result<(), String> + Sync)>,
     log: &(dyn Fn(&str) + Sync),
 ) -> Result<String, String> {
-    super::api::call_api_with_tools_sync(&def.agent, prompts, def.temperature, log)
+    super::api::call_api_with_tools_sync(&def.agent, prompts, def.temperature, bail_fn, log)
 }
 
 /// Parse a JSON response, handling markdown fences.