Prevent duplicate entries in `.gitignore` and `.cursorignore` during creation

[chasefleming]

Description:

Currently, when running the script that generates .gitignore and .cursorignore, it appends entries even if they already exist in the file. This results in duplicate entries being added repeatedly each time the script is run.

Expected Behavior:

• The script should only add new entries if they don’t already exist in the respective ignore files.
• If the file already contains the desired ignore entry, it should not add it again.
• If the file contains other unrelated entries, those should remain untouched—no deletions or overrides.

Steps to Reproduce:

1. Run the generator once — entries are added to .gitignore and .cursorignore.
2. Run it again — the same entries are appended again, creating duplicates.

Acceptance Criteria:

• No duplicate lines are added to .gitignore or .cursorignore.
• Existing unrelated entries in the files are preserved.
• Script is idempotent—running it multiple times yields the same result.

Additional Context:

This would improve clarity and prevent unnecessary file noise or git diff clutter.