feat(doctor): first-boot pre-activate fail flag (#83)

If the installer left /var/log/nomarchy-hm-preactivate.log and no Home
Manager generation exists, doctor reports ✖ with the recovery one-liner
(home-manager switch --flake ~/.nomarchy -b bak). Clears once a
generation is present. checks.doctor covers both arms.

Verified: V2 — checks.doctor green; local smoke.

flake.nix

@@ -461,6 +461,27 @@
                   "NOMARCHY_POWER_SUPPLY_ROOT=/tmp/fake-ps nomarchy-doctor 2>&1"
               )
               assert "MOUSE0" not in bat2, f"doctor reported Device-scope battery:\n{bat2}"
+              # #83 first-boot pre-activate fail flag: log present, no HM gen → ✖.
+              machine.succeed(
+                  "mkdir -p /var/log && "
+                  "echo 'error: activation failed' > /var/log/nomarchy-hm-preactivate.log"
+              )
+              # Bare test VM has no per-user HM profile for root.
+              pre = machine.fail("nomarchy-doctor 2>&1")
+              assert "pre-activat" in pre.lower() or "Home Manager generation" in pre, (
+                  f"doctor missing pre-activate fail line:\n{pre}"
+              )
+              assert "home-manager switch" in pre, f"doctor missing recovery one-liner:\n{pre}"
+              # Once a generation exists, the flag clears (recovered).
+              machine.succeed(
+                  "mkdir -p /nix/var/nix/profiles/per-user/root && "
+                  "ln -sfn /run/current-system "
+                  "/nix/var/nix/profiles/per-user/root/home-manager"
+              )
+              pre_ok = machine.succeed("nomarchy-doctor 2>&1")
+              assert "desktop generation is present" in pre_ok, (
+                  f"doctor should ok when HM gen exists:\n{pre_ok}"
+              )
             '';
           };