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.

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