fix: actionable remedy for root-created per-user config dir

A pmg run as root with a preserved HOME (GitHub runners, sudo -E, su
without -) creates the invoking user's ~/.config/safedep as root-owned,
and event-log init then fail-closes every later non-root command.
Make that state self-solvable:

- event-log init permission errors exit with a usefulerror naming the
  likely cause and the chown fix instead of a bare fatal
- pmg setup doctor probes event-log dir writability and reports the
  same fix via a new per-result Fix override
- document the mechanism and remedy in system-install.md, along with
  the binary ownership requirements for --system
- consolidate this branch's doctor tests into doctor_test.go

internal/doctor/doctor.go

@@ -14,6 +14,8 @@ type CheckResult struct {
 	Status              CheckStatus
 	Message             string
 	ImpliesInterception bool
+	// Fix overrides the check's static fix hint for this specific result.
+	Fix string
 }
 
 type Check struct {