feat: Implement write_file tool with overwrite safety (#3432)

## Summary

Implements the `write_file` client-side tool — creates new files or
overwrites existing ones with an explicit `overwrite` flag for safety.

- **Overwrite flag**: Writing to an existing file without `overwrite:
true` returns an error directing the LLM to set the flag or use
`edit_file` for targeted changes. Prevents accidental overwrites.
- **Snapshots**: Existing files are backed up before overwriting (same
infrastructure as `edit_file`).
- **Content preview**: Completed writes show the first 10 lines in gray
with line numbers, plus "+ N more lines" for longer files.
- **Atomic writes**: Uses `tempfile` + fsync + rename (same as
`edit_file`).
- **File tracker update**: After writing, the file is registered in the
tracker so subsequent `edit_file` calls work without a separate read.
- **Permission**: Shares the `"Write"` rule with `edit_file` — one
permission covers both tools.

2 files changed, 64 insertions, 10 deletions

diff --git a/crates/atuin-ai/src/tui/view/mod.rs b/crates/atuin-ai/src/tui/view/mod.rs
index bdbece9c..6e13e406 100644
--- a/crates/atuin-ai/src/tui/view/mod.rs
+++ b/crates/atuin-ai/src/tui/view/mod.rs
@@ -175,7 +175,7 @@ fn tool_call_view(tool_call: &TrackedTool, in_git_project: bool) -> Elements {
 /// keep the standard set.
 fn permission_options_for_tool(tool: &ClientToolCall, in_git_project: bool) -> Vec<SelectOption> {
     match tool {
-        ClientToolCall::Edit(_) => vec![
+        ClientToolCall::Edit(_) | ClientToolCall::Write(_) => vec![
             SelectOption::builder()
                 .label("Allow")
                 .value(PermissionResult::Allow.as_value_str())
@@ -296,8 +296,8 @@ fn agent_turn_view(events: &[turn::UiEvent], busy: bool) -> Elements {
                                     turn::ToolRenderData::FileEdit { path, preview } => {
                                         file_edit_tool_view(&tool_key, &details.status, path, preview.as_ref())
                                     },
-                                    turn::ToolRenderData::FileWrite { path } => {
-                                        file_write_tool_view(&details.status, path)
+                                    turn::ToolRenderData::FileWrite { path, preview } => {
+                                        file_write_tool_view(&tool_key, &details.status, path, preview.as_ref())
                                     },
                                     turn::ToolRenderData::Remote => {
                                         tool_status_view(&details.name, &details.status)
@@ -577,10 +577,16 @@ fn file_edit_tool_view(
     }
 }
 
-/// Render a file write tool call status with the target path.
-fn file_write_tool_view(status: &turn::ToolResultStatus, path: &std::path::Path) -> Elements {
-    let display_path = path.display();
-    match status {
+/// Render a file write tool call with content preview.
+fn file_write_tool_view(
+    key: &str,
+    status: &turn::ToolResultStatus,
+    path: &std::path::Path,
+    preview: Option<&crate::diff::WritePreview>,
+) -> Elements {
+    let display_path = format_path_for_display(path);
+
+    let status_line = match status {
         turn::ToolResultStatus::Pending => {
             element! {
                 Spinner(
@@ -591,18 +597,62 @@ fn file_write_tool_view(status: &turn::ToolResultStatus, path: &std::path::Path)
             }
         }
         turn::ToolResultStatus::Success => {
+            let line_info = preview
+                .map(|p| format!(" ({} lines)", p.total_lines))
+                .unwrap_or_default();
             element! {
-                Spinner(label: format!("Wrote: {display_path}"), done: true)
+                Spinner(label: format!("Wrote: {display_path}{line_info}"), done: true)
             }
         }
         turn::ToolResultStatus::Error => {
             element! {
                 Text {
                     Span(text: "✗ ", style: Style::default().fg(Color::Red))
-                    Span(text: format!("Write {display_path}: denied"), style: Style::default().fg(Color::Red))
+                    Span(text: format!("Write {display_path}: failed"), style: Style::default().fg(Color::Red))
                 }
             }
         }
+    };
+
+    let Some(preview) = preview else {
+        return status_line;
+    };
+    if preview.lines.is_empty() {
+        return status_line;
+    }
+
+    let gutter_width = preview.total_lines.to_string().len().max(2) as u16 + 1;
+    let remaining = preview.remaining_lines();
+
+    element! {
+        View(key: key.to_string()) {
+            #(status_line)
+
+            View(key: format!("{key}-content"), padding_left: Cells::from(2)) {
+                #(for (idx, line) in preview.lines.iter().enumerate() {
+                    HStack(key: format!("{key}-line-{idx}")) {
+                        View(width: WidthConstraint::Fixed(gutter_width)) {
+                            Text { Span(
+                                text: format!("{:>width$}", idx + 1, width = (gutter_width - 1) as usize),
+                                style: Style::default().fg(Color::DarkGray)
+                            ) }
+                        }
+                        View {
+                            Text { Span(text: line, style: Style::default().fg(Color::DarkGray)) }
+                        }
+                    }
+                })
+
+                #(if remaining > 0 {
+                    Text {
+                        Span(
+                            text: format!("     ... +{remaining} more lines"),
+                            style: Style::default().fg(Color::DarkGray)
+                        )
+                    }
+                })
+            }
+        }
     }
 }
 
diff --git a/crates/atuin-ai/src/tui/view/turn.rs b/crates/atuin-ai/src/tui/view/turn.rs
index 1c19a6b2..6c3d5c29 100644
--- a/crates/atuin-ai/src/tui/view/turn.rs
+++ b/crates/atuin-ai/src/tui/view/turn.rs
@@ -141,7 +141,10 @@ pub(crate) enum ToolRenderData {
         preview: Option<crate::diff::EditPreview>,
     },
     /// File write/create operation.
-    FileWrite { path: PathBuf },
+    FileWrite {
+        path: PathBuf,
+        preview: Option<crate::diff::WritePreview>,
+    },
     /// Atuin history search.
     HistorySearch {
         query: String,
@@ -449,6 +452,7 @@ impl<'a> TurnBuilder<'a> {
                 },
                 ClientToolCall::Write(write) => ToolRenderData::FileWrite {
                     path: write.path.clone(),
+                    preview: tracked.write_preview.clone(),
                 },
                 ClientToolCall::AtuinHistory(history) => ToolRenderData::HistorySearch {
                     query: history.query.clone(),