chore: standardise error message capitalisation

[adamdaw]

`validate-artifacts.sh` uses `ERROR:` (all caps) while all other scripts use `Error:` (title case). Standardise to `Error:` for consistency.

Files affected

• scripts/validate-artifacts.sh (err() function, line ~49)

Acceptance Criteria

• All error messages use `Error:` prefix
• All warning messages use `Warning:` or `WARN:` consistently