Add edit_file tool for surgical code edits

Summary

Add an edit_file tool that performs surgical edits on files — string replacement or line-range modifications — without overwriting the entire file.

Motivation

The existing write_file tool overwrites entire files, which is destructive for code changes. A coding agent needs to make targeted edits: replace a function signature, fix a specific line, insert a block at a given location. This is a prerequisite for using DocsClaw as a coding/code-analysis agent in CI/CD pipelines.

Proposed interface

{
  "name": "edit_file",
  "parameters": {
    "path": "string — file path (workspace-scoped)",
    "old_string": "string — exact text to find and replace",
    "new_string": "string — replacement text"
  }
}

Alternative mode (line-range):

{
  "parameters": {
    "path": "string",
    "start_line": "integer",
    "end_line": "integer",
    "content": "string — replacement content for the line range"
  }
}

Implementation notes

• Reuse workspace scoping and path traversal protection from write_file
• Read existing file, apply edit, write back
• Return diff preview in tool output (so the LLM can verify)
• Fail if old_string is not found or not unique (require enough context)
• Follow existing tool pattern: implement Tool interface, register in serve.go

Security considerations

• Same workspace boundary enforcement as read_file/write_file
• Symlink resolution before edit
• No binary file editing

Related

• Part of the coding agent capabilities gap analysis
• Pairs with grep (#27) and glob (#28) tools for code navigation + editing workflow

Start by reading the existing read_file and write_file implementations, the Tool interface, and registration in serve.go. Reuse the documented workspace and symlink protections, then verify that edits reject missing or non-unique text, avoid binary files, and return a diff preview. Done means the edit_file tool supports the agreed edit mode and is registered for use.