docs: two install shapes, two spellings, neither assuming `collie` on PATH

The update instructions were written for a name that most installs do not
have. A Herdr-managed install never gets a `collie` on PATH — Herdr owns the
checkout, and a GitHub install keeps it under a hashed path — so a page that
opens with `collie update` hands its largest group of readers a command that
cannot run. That group is exactly the 0.x population, since 0.x had no binary
and no install script at all.

So both docs now name the shape before the verb:

- upgrading.md opens on the update, not the uninstall, and every command comes
  in both spellings. The 0.x crossing is one section instead of two, with the
  BUN_INSTALL check in front of it. The v1 beta train is over, so that section
  is now a short prerelease note.
- install.md is two ways in — fresh install, or through Herdr — instead of
  four, with the multiplexer as the shared step it always was, and it ends
  with the update command it never carried.

Standalone verbs are spelled `bin/collie <verb>` throughout: the binary is
executable as it lands, so the full path always works, and `link` is an
option rather than a precondition.