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.

1 files changed, 5 insertions, 1 deletions

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(),