v0.20.2 — Refuse a core.worktree redirect of the workspace (#76)

Patch release: one refusal that no honest git layout hits, closing the route
by which a session could steer the next host-side launch to another directory.

Security

• A planted core.worktree can no longer redirect the next launch (#76,
  reproduced and closed). tjor resolves the workspace on the host with
  git rev-parse --show-toplevel. With core.worktree = <path> in a repo's
  .git/config, git answers <path> from anywhere inside that repo — and a
  session always has its workspace repo mounted writable, so it can plant
  that line. On the operator's next tjor run from the same repo, tjor
  silently adopted <path> as the workspace whenever it was not in the
  sensitive set (another project, or a parent holding every repo): mounted
  writable, git-trusted as a tree, under a session id derived from it. A
  redirect toward $HOME was caught by the v0.19.0 gate but reported as a
  git climb, and down/status/reset under any redirect acted on the
  wrong session. The launcher now finds the repository on its own — the
  nearest ancestor of the launch directory holding a .git entry — and
  requires git's reported work tree to be that directory (both physical);
  containment alone would not do, since a redirect to an ancestor still
  contains the launch directory. Every honest resolution agrees: plain
  repos, subdirectories, linked worktrees, submodules, nested repos,
  symlinked spellings. Otherwise it refuses, on the launch path and on every
  lifecycle path, naming the launch directory, the reported work tree, the
  repository it found, the core.worktree value and its config file, and
  the remedies. No override:
  a repo whose work tree is elsewhere (a detached-git-dir layout) is never a
  tjor workspace — launch from the real work tree; the qualified --session
  id keeps working from any other directory. The reproduction is a permanent
  regression section of the workspace-gate suite (one boundary-matrix
  row). Companion: #72 (detecting other cage-written git metadata that runs
  on the host).