session: add TranscriptInfo struct, consolidate transcript lookups

TranscriptInfo provides cached transcript metadata (path, size)
with a single read. Replaces scattered fs::metadata calls in
surface_observe_cycle, reflection_cycle, resolve_conversation,
and resolve_memory_ratio.

Session::transcript() resolves the path from transcript_path or
by searching projects dir, returning a TranscriptInfo.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/session.rs

@@ -1,7 +1,10 @@
-// session.rs — Session state for ambient hooks and agent interactions
+// session.rs — Session state and transcript info
 //
-// Tracks per-session state (seen set, state directory) across hook
+// Session: per-session state (seen set, state directory) across hook
 // invocations. Created from hook JSON input or POC_SESSION_ID env var.
+//
+// TranscriptInfo: cached metadata about the current session's transcript
+// file — size, path, compaction offset. Read once, used by all callers.
 
 use std::collections::HashSet;
 use std::fs;
@@ -53,4 +56,51 @@ impl Session {
     pub fn seen(&self) -> HashSet<String> {
         super::hippocampus::memory_search::load_seen(&self.state_dir, &self.session_id)
     }
+
+    /// Get transcript metadata, resolving the path if needed.
+    pub fn transcript(&self) -> TranscriptInfo {
+        if !self.transcript_path.is_empty() {
+            return TranscriptInfo::from_path(&self.transcript_path);
+        }
+
+        // Find transcript by session ID in projects dir
+        let projects = crate::config::get().projects_dir.clone();
+        if let Ok(dirs) = fs::read_dir(&projects) {
+            for dir in dirs.filter_map(|e| e.ok()) {
+                let path = dir.path().join(format!("{}.jsonl", self.session_id));
+                if path.exists() {
+                    return TranscriptInfo::from_path(&path.to_string_lossy());
+                }
+            }
+        }
+
+        TranscriptInfo::empty()
+    }
+}
+
+/// Cached transcript metadata — read once, use everywhere.
+pub struct TranscriptInfo {
+    pub path: String,
+    pub size: u64,
+}
+
+impl TranscriptInfo {
+    pub fn from_path(path: &str) -> Self {
+        let size = fs::metadata(path).map(|m| m.len()).unwrap_or(0);
+        TranscriptInfo {
+            path: path.to_string(),
+            size,
+        }
+    }
+
+    pub fn empty() -> Self {
+        TranscriptInfo {
+            path: String::new(),
+            size: 0,
+        }
+    }
+
+    pub fn exists(&self) -> bool {
+        !self.path.is_empty() && self.size > 0
+    }
 }