refactor: simplify card validation and option parsing

[marcalexiei]

• Replace supportedCoreExports, the handler map and the required-option switch with a single CARDS map (card type → core export + required option); adding a card is now a one-line change.
• Replace createCardHandlers + validateCardOptions with resolveCardHandler, which validates the card type, core exports and required option, then returns the handler.
• Fold normalizeOptions into parseOptions, using URLSearchParams.getAll(key).join(",") instead of manual duplicate-key merging.
• Move the username → GITHUB_REPOSITORY_OWNER fallback into run(), since it's action behavior rather than card logic.

[marcalexiei]

Hi @martin-mfg,
Could I get a review on this when you have a moment?
It's independent of my other open PR, so the two can be merged in either order.