feat(info): show the install reason and drop the indentation

N? and `info PKG` now print three lines, none indented:

    PKG: <description>
    reason: explicitly installed | installed as a dependency | unknown | not installed
    required by: <installed dependents, comma separated> | nothing

The reason comes from a new <be>_reason backend function: apt-mark
showauto / showmanual (dpkg decides for a package in neither list), the
Install Reason of pacman -Qi, dnf repoquery --installed %{reason} (dnf4
and dnf5 words) and the Installed line of zypper info (rpm plus libzypp's
AutoInstalled when zypper is locked). A row whose old version is "-" is
not installed and the backend is not asked. The "(installed)" qualifier
moves to --help, README and DESIGN.

The rpm E2E fixture and e2e_reset now also mark su-dummy-a and su-dummy-c
as user-installed on dnf, which records plain rpm installs as "unknown",
so 1? and 2? show both reasons on all 12 distros. The verified commands
are recorded in discovery/{apt,pacman,rpm}.md.

tests/unit/root.bats

@@ -39,7 +39,8 @@ setup() {
 	assert_line "apt${T}su-dummy-a${T}user${T}2026-10-09T09:40:12Z"
 	run skup info su-dummy-b
 	assert_success
-	assert_line '  required by (installed): su-dummy-a'
+	assert_line 'reason: installed as a dependency'
+	assert_line 'required by: su-dummy-a'
 	run skup show su-dummy-a
 	assert_success
 	assert_line 'Package: su-dummy-a'