session: extract Session from memory_search to src/session.rs

Generic session state (session_id, seen set, state directory) doesn't
belong in the memory search module. Now at crate root, re-exported
from memory_search for backwards compatibility.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/session.rs

@@ -0,0 +1,52 @@
+// session.rs — Session state for ambient hooks and agent interactions
+//
+// Tracks per-session state (seen set, state directory) across hook
+// invocations. Created from hook JSON input or POC_SESSION_ID env var.
+
+use std::collections::HashSet;
+use std::fs;
+use std::path::PathBuf;
+
+pub struct Session {
+    pub session_id: String,
+    pub transcript_path: String,
+    pub hook_event: String,
+    pub state_dir: PathBuf,
+}
+
+impl Session {
+    pub fn from_json(input: &str) -> Option<Self> {
+        let state_dir = PathBuf::from("/tmp/claude-memory-search");
+        fs::create_dir_all(&state_dir).ok();
+
+        let json: serde_json::Value = serde_json::from_str(input).ok()?;
+        let session_id = json["session_id"].as_str().unwrap_or("").to_string();
+        if session_id.is_empty() { return None; }
+        let transcript_path = json["transcript_path"].as_str().unwrap_or("").to_string();
+        let hook_event = json["hook_event_name"].as_str().unwrap_or("").to_string();
+
+        Some(Session { session_id, transcript_path, hook_event, state_dir })
+    }
+
+    pub fn path(&self, prefix: &str) -> PathBuf {
+        self.state_dir.join(format!("{}-{}", prefix, self.session_id))
+    }
+
+    /// Load from POC_SESSION_ID environment variable
+    pub fn from_env() -> Option<Self> {
+        let session_id = std::env::var("POC_SESSION_ID").ok()?;
+        if session_id.is_empty() { return None; }
+        let state_dir = PathBuf::from("/tmp/claude-memory-search");
+        Some(Session {
+            session_id,
+            transcript_path: String::new(),
+            hook_event: String::new(),
+            state_dir,
+        })
+    }
+
+    /// Get the seen set for this session
+    pub fn seen(&self) -> HashSet<String> {
+        super::hippocampus::memory_search::load_seen(&self.state_dir, &self.session_id)
+    }
+}