[FEATURE] Configurable context lines for Edit tool diffs

Problem

When the Edit tool modifies a file, the diff shown to the user has a fixed number of context lines. When edits are in code with repetitive structure (e.g., multiple similar blocks), the limited context can make it hard to tell which block was modified, leading to misreading the diff.

Proposal

Add a setting (e.g., in settings.json) to control the number of context lines shown around Edit tool diffs:

{
  "editDiffContextLines": 5
}

Default could remain as-is; users who find themselves misreading diffs due to elided lines could increase it.

Why

In a code review session, the user misread diffs twice due to insufficient context, mistakenly thinking code was missing when it was just outside the visible window. More context lines would have prevented both misreadings.

🤖 Filed with assistance from Claude Code.