v0.21.3 — Worktree mounts: native layout only, explicit approval; git-check walks every root (v0.21.2 review)

Review follow-ups for v0.21.2 (external review; both Criticals confirmed
against the code — one by running the resolver against the forged layout —
before fixing).

Security

• A forged worktree structure can no longer mount an arbitrary host
  directory (v0.21.2 review, Critical). The #79 resolver verified that
  the private git directory linked back to the root, but the private
  directory itself could live inside the writable workspace with a
  commondir file aimed at any repository on the host — three plaintext
  files, all writable from the cage, "proving" a path the operator never
  named. Two layers now: the private directory must be
  <common>/worktrees/<name> inside the common directory git resolves, so
  the back-link is a file inside the target that only someone who can
  already write there could plant; and a common directory outside every
  mount root is mounted only with --allow-worktree-mount (or named with
  --dir), else the launch refuses and prints the exact path. tjor mounts
  only paths the operator named, again.
• git-check no longer skips a root on its shape (v0.21.2 review,
  Critical). HEAD + objects/ + refs/ planted at the top of the
  workspace made its whole subtree invisible to the baseline and every
  check, silently. Every root is walked; a git-directory root simply yields
  no repository of its own.
• A symlinked .git/config is refused regardless of the pin setting
  (v0.21.2 review, Low): the refusal was gated on protect_git_config
  while documented as unconditional.

Changed

• worktree_common_dir reports each git rev-parse failure with its own
  error text. README: the acknowledgement token binds to a state, not to
  proof of review — keep --ack in human hands; two sessions sharing a
  main repository coordinate only through git's own locks. Tests: the
  forged layout, the unapproved common dir, explicit --dir approval, the
  planted git-dir shape, the config symlink with the pin off.