Add --list-targets to the validation script

[jeleel-muibi]

Area
Tooling

Problem
scripts/validate.sh supports named targets, but new contributors have to read the README or inspect clusters/ to know which targets are available. A simple listing mode would make local validation easier to discover.

Proposed change
Add a --list-targets option to scripts/validate.sh.

Suggested behaviour:

• print available target names from clusters/
• do not require a target to be selected
• exit successfully after printing the list
• include the option in the help output

Acceptance criteria

• ./scripts/validate.sh --list-targets prints the available targets.
• ./scripts/validate.sh --help mentions the option.
• Existing validation behaviour is unchanged.