[Good First Issue] Add gitignore entries for local generated dirs

[Gupta91]

Problem

Local generated directories/files such as .diffshub/, .lavish/, and skills-lock.json can appear as untracked files.

Expected Behavior

Review local generated artifacts and add appropriate .gitignore entries without hiding source files that should be committed.

Acceptance Criteria

• .gitignore covers confirmed local-only generated artifacts.
• Source/config files that should be versioned remain visible.
• The change is documented briefly in the PR description.