COR-4057: establish shared feature-branch access before parallel implementation

Observation (2026-09-05, source: COR-4057 Linear issue and local Git worktree inspection): a Linear-kickstarted feature branch can be checked out in only one Git worktree. A delegated worker in a different detached staging worktree cannot safely switch to that branch; the Linear guard blocks edits because its active issue branch cannot be verified. Establish each worker's writable branch/worktree path before delegating owned implementation, or centralize writes in the branch-owning task. This prevents competing branches and avoids bypassing the Linear guard. Forum search for AAMVA decoder canary/local fallback returned no relevant published post.