Reach other repos with a -C <path> directory flag, not a hub

Forest checkout workspaces are per-repo and self-contained (walk-up halts at the first .forest/ or .git; see ADR-0010). To operate on another repo without changing directory, forest accepts a git-style -C <path> flag that sets the discovery working directory before the normal walk-up + HEAD resolution runs. There is no parent-registers-children "hub": G004 can point at a sibling repo with -C, but never registers or drives it implicitly.

-C is distinct from the removed global --config (ADR-0004): --config named a forest.yaml file and bypassed discovery; -C only changes the starting directory, so the single-active-config invariant holds.

Consequences

• forest -C ../bcl2airr status resolves bcl2airr's own workspace and HEAD.
• Cross-repo orchestration is shell-level (for d in ...; do forest -C "$d" ...), not a forest feature.
• A checkout is trial-granular; a new sequencing run is a push of a new unit, never a checkout.