update/upgrade: auto-detect flake vs channel and use correct path — nina upgrade now updates flake inputs (exec.flakeUpdateAt: cd config dir && nix flake update) on a flake system instead of nix-channel --update, then rebuilds (rebuild already flake-aware); channel path unchanged for channel systems. NOTE: in this codebase 'nina update' is the kepr binary self-updater, so the channel-vs-flake branch lives in 'upgrade' (the channel-update+rebuild command); flake-system note for plain channel ops is shown by 'nina channel'. build+tests green

@@ -460,8 +460,16 @@460460 461461 pub fn upgrade(ctx: Ctx) !void {462462     output.upgradeStart(ctx.machine.name);463-    output.printSubstep("updating channels", .{});464-    _ = exec.channelUpdate(ctx.io, &ctx.machine, ctx.gpa) catch {};463+    // Flake systems update inputs (flake.lock); channel systems update channels.464+    // Running nix-channel --update on a flake system is a no-op at best. The465+    // rebuild below is already flake-aware via exec.nixosRebuildSwitch.466+    if (flakeSystem(ctx)) {467+        output.printSubstep("updating flake inputs", .{});468+        _ = exec.flakeUpdateAt(ctx.io, &ctx.machine, ctx.gpa, ctx.machine.config_path) catch {};469+    } else {470+        output.printSubstep("updating channels", .{});471+        _ = exec.channelUpdate(ctx.io, &ctx.machine, ctx.gpa) catch {};472+    }465473     output.printSubstep("rebuilding", .{});466474     const start = std.Io.Clock.awake.now(ctx.io);467475     const code = try exec.nixosRebuildSwitch(ctx.io, &ctx.machine, ctx.gpa);

@@ -744,6 +744,13 @@744744     return stream(io, machine, gpa, &.{ "nix", "flake", "update" });745745 }746746 747+// Update flake inputs for the flake living in `dir` (the config dir). `nix flake748+// update` operates on the flake in the working directory, so we cd there first.749+// Used by `nina upgrade` on a flake system in place of nix-channel --update.750+pub fn flakeUpdateAt(io: std.Io, machine: *const types.Machine, gpa: std.mem.Allocator, dir: []const u8) !u8 {751+    return stream(io, machine, gpa, &.{ "sh", "-c", "cd \"$0\" && nix flake update", dir });752+}753+747754 pub fn flakeCheck(io: std.Io, machine: *const types.Machine, gpa: std.mem.Allocator) !u8 {748755     return stream(io, machine, gpa, &.{ "nix", "flake", "check" });749756 }