GET …/forks?tree=1 answers the whole fork tree in one call, with drift for the first ranked forks #11087

Intent: the HTTP half of #10509 (parked once): an agent reads the tree without walking it.

Done:

• [ ] GET /api/v1/repos/:owner/:name/forks?tree=1 in src/http/api/upstream.ts answers the forkTree shape from src/actions/fork_tree.ts, with ahead/behind for at most MAX_RANKED_FORKS nodes (null past it), limit/cursor/next_cursor as the flat list has
• [ ] without tree=1 the flat answer is byte-for-byte unchanged (a test pins it)
• [ ] test/fork-tree-api.test.ts reads a three-level tree and a private grandchild is absent for a stranger
• [ ] the route's contract entry names tree so /contract/openapi.json lists it

Files: src/http/api/upstream.ts, the route's contract entry, test/fork-tree-api.test.ts Suits: torque