v0.7.0

Fix Linux and Windows test runs

- kopia.rs: `--no-use-keychain` exists only in the macOS build of Kopia
  0.23.1; Linux (and Windows) reject it and every e2e test failed at
  `init`. Pass it on macOS only. Also fall back to `kopia.exe` on
  Windows when PATHEXT is absent from the environment.
- contain/unix.rs: `mode_t` casts are needed on macOS and flagged as
  redundant on Linux; allow the lint on that one binding.
- contain/mod.rs test: Linux RESOLVE_IN_ROOT re-roots a `..` chain back
  inside the root rather than refusing it; accept that, the on-disk
  checks still prove nothing escaped.
- place.rs test: pick a source OS that differs from the host so the
  cross-OS symlink skip fires on Linux too.
- cli_e2e.rs: on Windows pass PATHEXT, SYSTEMROOT and the temp dirs to
  the child and set USERPROFILE to the fixture home.
- docs/kopia-compat.md: note the keychain flag is macOS only.

Verified locally on macOS and in a Linux container (fmt, clippy, all
tests with Kopia 0.23.1) plus clippy for the x86_64-pc-windows-msvc
target.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>