Remove global --config

Forest checkout workspaces use one active config resolved from .forest/config.yaml plus .forest/HEAD, with legacy fallback to ./forest.yaml when no workspace exists. The global --config escape hatch is removed because multiple config entry points make checkout behavior harder to reason about and no existing users require compatibility.

Amendment (Grilling Round 2)

A -C <path> directory flag is reintroduced (ADR-0008) and does not reopen this decision. --config named a forest.yaml file and bypassed discovery; -C only changes the discovery working directory, after which the same single-active-config resolution runs. The "one active config" invariant this ADR protects is preserved.