…cs; close coverage gaps

Four review items from @AlexandreYang on PR #237 addressed in one
commit:

1. Promote the -P-above-sandbox passthrough invariant from a local
   comment into the package docstring so callers can reason about
   when cd -P diverges from bash on container-style sandboxes.

2. Improve the diagnostic emitted when ChangeDir is nil (the
   RunCommand-child case used by find -exec / -execdir / xargs).
   Was "cd: not supported in this runner" — generic and opaque to
   users. Now reads "cd: cannot change directory from inside
   find -exec/-execdir or xargs (child invocations are isolated)"
   so the user knows the cause is intentional isolation.

3. Add a scenario for HOME pointing outside the sandbox
   (errors/home_outside_sandbox.yaml). Existing scenarios only
   covered HOME unset and HOME empty; the rejection path where
   resolveOperand succeeds but changeDir rejects was untested.

4. Add four hardening unit tests for the HostPrefix re-prefix
   logic in resolveSymlinks: unrelated absolute target gets
   re-prefixed, already-prefixed target is left alone, target
   exactly equal to hp is left alone, and the
   /mnt/host vs /mnt/hostile prefix-collision boundary is
   correctly distinguished by the hp+sep check.

P3 review items (#5 Windows ACL test, #6 isCd literal-name
defer-skip) are intentionally not addressed — both are flagged
as not blocking and are forward-looking notes; replies posted
on the threads.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>