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/tool_dispatch.rs

@@ -7,7 +7,7 @@ use anyhow::Result;
 use tracing::{debug, warn};
 
 use crate::tools::executor::ToolContext;
-use crate::tools::{file_ops, memory, misc, research, shell, todo, webdriver};
+use crate::tools::{acd, file_ops, memory, misc, research, shell, todo, webdriver};
 use crate::ui_writer::UiWriter;
 use crate::ToolCall;
 
@@ -47,6 +47,9 @@ pub async fn dispatch_tool<W: UiWriter>(
         // Project memory tools
         "remember" => memory::execute_remember(tool_call, ctx).await,
 
+        // ACD (Aggressive Context Dehydration) tools
+        "rehydrate" => acd::execute_rehydrate(tool_call, ctx).await,
+
         // WebDriver tools
         "webdriver_start" => webdriver::execute_webdriver_start(tool_call, ctx).await,
         "webdriver_navigate" => webdriver::execute_webdriver_navigate(tool_call, ctx).await,