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/digest.rs

@@ -226,7 +226,7 @@ fn generate_digest(
     // Load prompt from agent file; fall back to prompts dir
     let def = super::defs::get_def("digest");
     let template = match &def {
-        Some(d) => d.prompts.first().cloned().unwrap_or_default(),
+        Some(d) => d.steps.first().map(|s| s.prompt.clone()).unwrap_or_default(),
         None => {
             let path = crate::config::get().prompts_dir.join("digest.md");
             std::fs::read_to_string(&path)