One board, step 1: a card says which board it came from and carries one identity #1277

Written-by: Dev4

Step 1 of #1265, split out so the umbrella stays open for steps 2–4 (the triage comment on #1265 asks for four pull requests, and enqueue-guard needs the checkpoint's issue to be one this body closes, which a Part of #1265 body cannot name).

What this step is

One card model behind the two boards: a card carries source: 'github' | 'task' and a key that is unique across both — owner/name#number for a repository card, task:<id> for a FlyBot task record — and one component draws either, with each board's own class names passed as a skin.

owner/name#number is deliberately not keyed by kind: an issue and the pull request that closes it are one card, which is the identity step 3 hangs a record under. A row with no repository falls back to github:<kind>:<number>.

Not this step

Columns stay as they are (step 2), records are not linked under issue cards yet (step 3), and one repository still (step 4).

Done when

core/src/shared/board-card.js and core/src/client/board-card.js exist, both boards render through the one component with data-source / data-key, and core/test/board-card.test.js proves the identity rules. Pull request links here.