llm: prevent orphaned subprocesses with PR_SET_PDEATHSIG

When the daemon is killed, spawned claude CLI processes survived as
orphans and burned CPU indefinitely. Use pre_exec to set
PR_SET_PDEATHSIG(SIGTERM) so children die with their parent.

Also fix byte-index truncation of stderr preview (same UTF-8 panic
pattern fixed in the daemon).

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/llm.rs

@@ -1,27 +1,38 @@
 // LLM utilities: model invocation and response parsing
 //
-// Shared by digest, audit, enrich, consolidate, knowledge, and fact_mine.
+// Calls claude CLI as a subprocess. Uses prctl(PR_SET_PDEATHSIG)
+// so child processes die when the daemon exits, preventing orphans.
 
 use crate::store::Store;
 
 use regex::Regex;
 use std::fs;
+use std::os::unix::process::CommandExt;
 use std::process::Command;
 
 /// Call a model via claude CLI. Returns the response text.
+///
+/// Sets PR_SET_PDEATHSIG on the child so it gets SIGTERM if the
+/// parent daemon exits — no more orphaned claude processes.
 fn call_model(model: &str, prompt: &str) -> Result<String, String> {
     // Write prompt to temp file (claude CLI needs file input for large prompts)
-    // Use thread ID + PID to avoid collisions under parallel rayon calls
     let tmp = std::env::temp_dir().join(format!("poc-llm-{}-{:?}.txt",
         std::process::id(), std::thread::current().id()));
     fs::write(&tmp, prompt)
         .map_err(|e| format!("write temp prompt: {}", e))?;
 
-    let result = Command::new("claude")
+    let result = unsafe {
+        Command::new("claude")
             .args(["-p", "--model", model, "--tools", "", "--no-session-persistence"])
             .stdin(fs::File::open(&tmp).map_err(|e| format!("open temp: {}", e))?)
             .env_remove("CLAUDECODE")
-        .output();
+            .pre_exec(|| {
+                // Kill this child if the parent dies
+                libc::prctl(libc::PR_SET_PDEATHSIG, libc::SIGTERM);
+                Ok(())
+            })
+            .output()
+    };
 
     fs::remove_file(&tmp).ok();
 
@@ -31,7 +42,8 @@ fn call_model(model: &str, prompt: &str) -> Result<String, String> {
                 Ok(String::from_utf8_lossy(&output.stdout).trim().to_string())
             } else {
                 let stderr = String::from_utf8_lossy(&output.stderr);
-                Err(format!("claude exited {}: {}", output.status, stderr.trim()))
+                let preview: String = stderr.chars().take(500).collect();
+                Err(format!("claude exited {}: {}", output.status, preview.trim()))
             }
         }
         Err(e) => Err(format!("spawn claude: {}", e)),
@@ -75,7 +87,8 @@ pub(crate) fn parse_json_response(response: &str) -> Result<serde_json::Value, S
         }
     }
 
-    Err(format!("no valid JSON in response: {}...", &cleaned[..cleaned.len().min(200)]))
+    let preview: String = cleaned.chars().take(200).collect();
+    Err(format!("no valid JSON in response: {preview}..."))
 }
 
 /// Get semantic keys (non-journal, non-system) for prompt context.