Hybrid engine

Furca does not pick one strategy for every git operation. Reads — log, status, diff, blame, refs — run in-process through gitoxide, because a read cannot corrupt a repository and its speed is felt on every interaction. Mutations — commit, merge, rebase, push, pull, stash — run through the system git CLI, because their correctness depends on hooks, credential helpers, LFS filters and commit signing already configured on the machine.

See ADR 0001 for the full reasoning.