feat(mcp): structured tool outputs and delete mode (#4)

* docs(tools): clarify delete and CLI alternatives

* feat(edit): add mode "delete" shorthand

* feat(mcp): structured outputs via outputSchema

Tools return structuredContent matching the declared outputSchema:

- read_anchored: total_lines + anchored_text
- edit_anchored: total_lines + affected_lines + unified-diff block
  with " "/"-"/"+" prefixes, anchored per line, and 3 lines of context
- write_to_file: total_lines + Myers-added runs only (unchanged lines
  retain prior anchors)

EditResult gains an operation discriminant, a nullable originalRange,
and a pre-edit snapshot consumed by buildDiff.