feat: trigger observation agent on conversation volume

The hook now tracks transcript size and queues an observation agent
run every ~5K tokens (~20KB) of new conversation. This makes memory
formation reactive to conversation volume rather than purely daily.

Configurable via POC_OBSERVATION_THRESHOLD env var. The observation
agent's chunk_size (in .agent file) controls how much context it
actually processes per run.

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

poc-memory/src/bin/poc-hook.rs

@@ -18,6 +18,16 @@ use std::time::{SystemTime, UNIX_EPOCH};
 const CONTEXT_THRESHOLD: u64 = 900_000;
 const RATE_LIMIT_SECS: u64 = 60;
 const SOCK_PATH: &str = ".claude/hooks/idle-timer.sock";
+/// How many bytes of new transcript before triggering an observation run.
+/// Override with POC_OBSERVATION_THRESHOLD env var.
+/// Default: 20KB ≈ 5K tokens. The observation agent's chunk_size (in .agent
+/// file) controls how much context it actually reads.
+fn observation_threshold() -> u64 {
+    std::env::var("POC_OBSERVATION_THRESHOLD")
+        .ok()
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(20_000)
+}
 
 fn now_secs() -> u64 {
     SystemTime::now()
@@ -73,6 +83,34 @@ fn check_notifications() {
     }
 }
 
+/// Check if enough new conversation has accumulated to trigger an observation run.
+fn maybe_trigger_observation(transcript: &PathBuf) {
+    let cursor_file = poc_memory::store::memory_dir().join("observation-cursor");
+
+    let last_pos: u64 = fs::read_to_string(&cursor_file)
+        .ok()
+        .and_then(|s| s.trim().parse().ok())
+        .unwrap_or(0);
+
+    let current_size = transcript.metadata()
+        .map(|m| m.len())
+        .unwrap_or(0);
+
+    if current_size > last_pos + observation_threshold() {
+        // Queue observation via daemon RPC
+        let _ = Command::new("poc-memory")
+            .args(["agent", "daemon", "run", "observation", "1"])
+            .stdout(std::process::Stdio::null())
+            .stderr(std::process::Stdio::null())
+            .spawn();
+
+        eprintln!("[poc-hook] observation triggered ({} new bytes)", current_size - last_pos);
+
+        // Update cursor to current position
+        let _ = fs::write(&cursor_file, current_size.to_string());
+    }
+}
+
 fn check_context(transcript: &PathBuf, rate_limit: bool) {
     if rate_limit {
         let rate_file = PathBuf::from("/tmp/claude-context-check-last");
@@ -175,6 +213,7 @@ fn main() {
 
             if let Some(ref t) = transcript {
                 check_context(t, false);
+                maybe_trigger_observation(t);
             }
         }
         "PostToolUse" => {