enhancement: implement edit tool (line-oriented editing)

[puremachinery]

Context

The current file mutation tools are `write_file` (full overwrite) and `replace` (exact string match replacement). The upstream Node.js CLI also provides an `edit` tool that supports line-range-based editing — inserting, deleting, or replacing specific line ranges.

This is a meaningful gap because:

• `replace` requires the model to reproduce the exact target string, which is fragile for large blocks
• `write_file` requires rewriting the entire file for a small change
• Line-oriented editing is a more natural fit for how models think about code modifications

Proposed change

Implement an `edit` tool in `internal/tools/` that accepts:

• `file_path`: target file
• `start_line` / `end_line`: line range to operate on
• `new_content`: replacement content for that range (empty = delete)
• `insert_before` or `insert_after`: insert mode (alternative to replace)

The tool should:

• Register in `internal/tools/registry.go`
• Use the same approval flow as `write_file` and `replace`
• Validate line numbers against actual file length
• Handle edge cases (empty file, single-line file, append at end)

Relevant files

• `internal/tools/registry.go` — tool registration
• `internal/tools/replace.go` — existing string-replace tool (pattern to follow)
• `internal/tools/executor.go` — approval flow