feat(#2196): move Write/Edit diff body to backend, drop JS branch

- fmt_builtin_tool: Edit gets its own arm computing `-N +M` line counts
  in _summary (was shared with Read/Write as bare file-path).
- is_rich_tool: drop Write (content is huge/one-sided; flat _summary row
  is correct); Edit stays rich since it has an old/new diff.
- rich_tool_body: now returns Option<(String, &'static str)> where the
  second field is the body type ('diff' or 'plain'). Edit arm builds the
  '-'/'+ ' prefixed diff body; mcp__bash__run gets type 'plain'.
- enrich_tool_use_entry: stamps both _body and _body_type.

Frontend (app.js):
- Remove the Write/Edit branch from renderRichToolUse (~25 lines).
- Generic _body path now dispatches on _body_type: 'diff' ->
  api.detailsDiff (colour-coded spans), default -> api.details.
  No tool-specific JS remains for file diff rendering.

hive-agent/src/stream_enrich.rs

@@ -239,8 +239,9 @@ fn enrich_tool_use_entry(entry: &mut Value) {
     if rich {
         obj.insert("_category".to_owned(), json!("rich"));
     }
-    if let Some(b) = body {
+    if let Some((b, bt)) = body {
         obj.insert("_body".to_owned(), json!(b));
+        obj.insert("_body_type".to_owned(), json!(bt));
     }
 }
 
@@ -255,8 +256,7 @@ fn enrich_tool_use_entry(entry: &mut Value) {
 fn is_rich_tool(name: &str) -> bool {
     matches!(
         name,
-        "Write"
-            | "Edit"
+        "Edit"
             | "mcp__bash__run"
             | "mcp__hyperhive__send"
             | "mcp__hyperhive__ask"
@@ -264,14 +264,42 @@ fn is_rich_tool(name: &str) -> bool {
     )
 }
 
-/// Pre-compute the expandable body text for tools that have one.
+/// Pre-compute the expandable body for rich tool entries.
 ///
-/// Returns `Some(body)` when the tool has a meaningful multi-line body that
-/// the frontend can display in a `<details>` block without tool-specific JS.
-/// `None` for tools whose body is computed client-side (diffs, markdown) or
-/// that have no body at all.
-fn rich_tool_body(name: &str, input: &Value) -> Option<String> {
+/// Returns `Some((body, body_type))` where `body_type` tells the frontend
+/// which renderer to use:
+/// - `"diff"` → `api.detailsDiff` (colour-coded `+`/`-` lines)
+/// - `"plain"` → `api.details` (plain `<pre>` block)
+///
+/// Returns `None` for tools whose body is inherently client-side (markdown
+/// rendering, DOM forms) or that have no body at all.
+fn rich_tool_body(name: &str, input: &Value) -> Option<(String, &'static str)> {
     match name {
+        // Edit diff: old lines prefixed `- `, new lines prefixed `+ `.
+        // The summary already carries `-N +M` counts (from `fmt_builtin_tool`),
+        // so the body is the full colour-coded diff.
+        "Edit" => {
+            let old = input
+                .get("old_string")
+                .and_then(Value::as_str)
+                .unwrap_or("");
+            let new = input
+                .get("new_string")
+                .and_then(Value::as_str)
+                .unwrap_or("");
+            let mut body = String::new();
+            for line in old.lines() {
+                body.push_str("- ");
+                body.push_str(line);
+                body.push('\n');
+            }
+            for line in new.lines() {
+                body.push_str("+ ");
+                body.push_str(line);
+                body.push('\n');
+            }
+            Some((body, "diff"))
+        }
         // Full bash command — first line is already in `_summary`; the full
         // `cmd` (prefixed with `$ `) is the body so multi-line scripts are
         // readable on expand.
@@ -280,7 +308,7 @@ fn rich_tool_body(name: &str, input: &Value) -> Option<String> {
             if cmd.is_empty() {
                 None
             } else {
-                Some(format!("$ {cmd}"))
+                Some((format!("$ {cmd}"), "plain"))
             }
         }
         _ => None,
@@ -365,7 +393,23 @@ fn fmt_tool_use(name: &str, input: &Value) -> String {
 /// unknown tool that doesn't carry a known MCP server prefix.
 fn fmt_builtin_tool(name: &str, short: &str, input: &Value) -> String {
     match name {
-        "Read" | "Write" | "Edit" => format!("{short} {}", sv(input, "file_path")),
+        "Read" | "Write" => format!("{short} {}", sv(input, "file_path")),
+        "Edit" => {
+            let path = sv(input, "file_path");
+            let old_n = input
+                .get("old_string")
+                .and_then(Value::as_str)
+                .unwrap_or("")
+                .lines()
+                .count();
+            let new_n = input
+                .get("new_string")
+                .and_then(Value::as_str)
+                .unwrap_or("")
+                .lines()
+                .count();
+            format!("{short} {path} · -{old_n} +{new_n}")
+        }
         "Glob" | "Grep" => format!("{short} {}", sv(input, "pattern")),
         "Bash" => {
             let bg = if input