Fix ACD turn summary loss and add /dump command

ACD (Aggressive Context Dehydration) fixes:
- Fixed dehydrate_context() to extract turn summary from context window
  instead of using the passed-in final_response (which contained only
  the timing footer, not the actual LLM response)
- Removed final_response parameter from dehydrate_context() since it
  now self-extracts the last assistant message as the summary
- This ensures the actual turn summary is preserved after dehydration,
  not just the timing footer

New /dump command:
- Added /dump command to dump entire context window to tmp/ for debugging
- Shows message index, role, kind, content length, and full content
- Available in both console and machine modes

UTF-8 safety:
- Fixed truncate_to_word_boundary() to use character indices instead of
  byte indices, preventing panics on multi-byte UTF-8 characters
- Added UTF-8 string slicing guidance to AGENTS.md

Agent: g3

crates/g3-core/src/paths.rs

@@ -85,6 +85,12 @@ pub fn get_thinned_dir(session_id: &str) -> PathBuf {
     get_session_logs_dir(session_id).join("thinned")
 }
 
+/// Get the fragments directory for a session (for ACD dehydrated context).
+/// Returns .g3/sessions/<session_id>/fragments/
+pub fn get_fragments_dir(session_id: &str) -> PathBuf {
+    get_session_logs_dir(session_id).join("fragments")
+}
+
 /// Get the path to the session.json file for a session.
 /// Returns .g3/sessions/<session_id>/session.json
 pub fn get_session_file(session_id: &str) -> PathBuf {