A new forge follows the topmost forge on its own: its forge repository takes updates by default, and a clean update merges itself #11143

Why. Owner: "the users forge will of course have the forge repo in their forge and it should automatically be wired up to sync automatically from forge.open-platform.sh/trevato/forge… all forges will update but the user can take it another direction should they so choose." Today the default followMode is tell-me (an inbox card per version) and nothing merges itself (src/spawn/upstream.ts, src/spawn/take.ts).

What. New forges default to take-updates. When the forge/take-updates pull has no conflict and the owner has not changed the repository themselves, it merges on its own; once the owner's fork has commits of its own, updates arrive as a pull to review, as now. The owner can switch to tell-me or sever the link from the repository's settings. Existing forges keep their current mode.

Acceptance. Tests: an untouched fork fast-forwards within one check; a fork with its own commits gets a pull and no automatic merge; a severed fork gets nothing.