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/hippocampus/memory_search.rs

@@ -8,7 +8,7 @@ use std::collections::HashSet;
 use std::fs;
 use std::fs::File;
 use std::io::Write;
-use std::path::{Path, PathBuf};
+use std::path::Path;
 use std::process::Command;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
@@ -19,49 +19,7 @@ fn now_secs() -> u64 {
 /// Max bytes per context chunk (hook output limit is ~10K chars)
 const CHUNK_SIZE: usize = 9000;
 
-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> {
-        load_seen(&self.state_dir, &self.session_id)
-    }
-}
+pub use crate::session::Session;
 
 /// Run the hook logic on parsed JSON input. Returns output to inject.
 pub fn run_hook(input: &str) -> String {
@@ -148,7 +106,7 @@ fn parse_seen_line(line: &str) -> &str {
     line.split_once('\t').map(|(_, key)| key).unwrap_or(line)
 }
 
-fn load_seen(dir: &Path, session_id: &str) -> HashSet<String> {
+pub fn load_seen(dir: &Path, session_id: &str) -> HashSet<String> {
     let path = dir.join(format!("seen-{}", session_id));
     if path.exists() {
         fs::read_to_string(&path)