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, 34 insertions, 0 deletions

diff --git a/crates/atuin-ai/src/diff.rs b/crates/atuin-ai/src/diff.rs
index 663481c0..e704175c 100644
--- a/crates/atuin-ai/src/diff.rs
+++ b/crates/atuin-ai/src/diff.rs
@@ -101,6 +101,40 @@ impl EditPreview {
     }
 }
 
+/// Maximum lines to show in a write preview.
+const WRITE_PREVIEW_LINES: usize = 10;
+
+/// A content preview for a write_file operation.
+///
+/// Shows the first N lines of the written content plus a count of
+/// remaining lines if truncated.
+#[derive(Debug, Clone)]
+pub(crate) struct WritePreview {
+    /// First lines of content (up to WRITE_PREVIEW_LINES).
+    pub lines: Vec<String>,
+    /// Total number of lines in the written file.
+    pub total_lines: usize,
+}
+
+impl WritePreview {
+    /// Create a preview from file content.
+    pub fn from_content(content: &str) -> Self {
+        let all_lines: Vec<&str> = content.lines().collect();
+        let total_lines = all_lines.len();
+        let lines = all_lines
+            .into_iter()
+            .take(WRITE_PREVIEW_LINES)
+            .map(String::from)
+            .collect();
+        WritePreview { lines, total_lines }
+    }
+
+    /// Number of lines not shown in the preview.
+    pub fn remaining_lines(&self) -> usize {
+        self.total_lines.saturating_sub(self.lines.len())
+    }
+}
+
 /// Build a single DiffHunk from a group of adjacent raw hunks.
 fn build_hunk(group: &[&imara_diff::Hunk], input: &InternedInput<&str>) -> DiffHunk {
     let first = group.first().unwrap();