ct/debian: load the engine from community-scripts/core

[MickLesk]

Pilot for the core split. The bootstrap no longer sources ../misc/build.func from this repo; it takes the engine from core, preferring a local checkout (COMMUNITY_SCRIPTS_CORE_DIR, else a sibling ../core) and falling back to COMMUNITY_SCRIPTS_CORE_URL.

Engine and scripts now resolve independently, so a fork or branch of core can be tested against these scripts without editing this file -- which is the workflow gap ProxmoxVED exists to work around.

misc/ stays in place: the other 94 ct scripts still source it. It can be removed once they are converted too.

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