surface agent infrastructure: hook spawn, seen set rotation, config

Surface agent fires asynchronously on UserPromptSubmit, deposits
results for the next prompt to consume.  This commit adds:

- poc-hook: spawn surface agent with PID tracking and configurable
  timeout, consume results (NEW RELEVANT MEMORIES / NO NEW), render
  and inject surfaced memories, observation trigger on conversation
  volume
- memory-search: rotate seen set on compaction (current → prev)
  instead of deleting, merge both for navigation roots
- config: surface_timeout_secs option

The .agent file and agent output routing are still pending.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/config.rs

@@ -67,6 +67,9 @@ pub struct Config {
     agent_model: Option<String>,
     pub api_reasoning: String,
     pub agent_types: Vec<String>,
+    /// Surface agent timeout in seconds. Kill if running longer than this.
+    #[serde(default)]
+    pub surface_timeout_secs: Option<u32>,
 }
 
 impl Default for Config {
@@ -105,6 +108,7 @@ impl Default for Config {
                 "linker".into(), "organize".into(), "distill".into(),
                 "separator".into(), "split".into(),
             ],
+            surface_timeout_secs: None,
         }
     }
 }