v0.5.0 — --repo flag + STITCH_REPO env var for out-of-repo invocation

Features

• Run stitch from outside the repo. A global --repo <path> flag and
  STITCH_REPO env var let you operate on a repo without cd-ing into it.
  Precedence: --repo > STITCH_REPO > upward cwd walk (unchanged default).
  An override must point at a directory containing .stitch/ — a typo is
  rejected, not silently applied to the wrong directory. init is cwd-anchored
  and ignores both (it creates a new repo in the current directory).