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/compaction.rs

@@ -191,6 +191,8 @@ pub async fn perform_compaction<W: UiWriter>(
     // Execute summary request
     match provider.complete(summary_request).await {
         Ok(summary_response) => {
+            // Note: ACD dehydration now happens at the end of each turn in Agent::dehydrate_context()
+            // Compaction just does lossy summarization of the existing stubs + summaries
             let chars_saved = context_window.reset_with_summary(
                 summary_response.content,
                 compaction_config.latest_user_msg,