fix: include --from git+... in upgrade hint to avoid PyPI squat package (#2411)

The compatibility-error messages in extensions.py and presets.py, plus the
extension troubleshooting guide, told users to upgrade with:

    uv tool install specify-cli --force

Without `--from git+https://github.com/github/spec-kit.git`, uv resolves
`specify-cli` from PyPI, where an unrelated package with the same name
(no author, no project URLs) ships a stub CLI that lacks `extension`,
`preset`, and most spec-kit commands. Users following the upgrade hint
land on the squat package and report "extension command removed"
(see #1982).

Reuse the existing `REINSTALL_COMMAND` constant in extensions.py and
import it from presets.py so all three call sites point at the GitHub
source. The doc fix also adds a one-line note explaining the PyPI
collision so the same advice doesn't get re-stripped later.

Refs #1982