feat(devshell): stream quickshell warnings and errors in nix develop

qs-log follows the working-tree instance's log filtered to WARN|ERROR (-a for
everything), and the shellHook starts it in the background once qs-dev is up,
taking it down again in the exit trap alongside qs-prod. A binding loop or a
failed binding is a WARN, and easy to miss when it scrolls past unwatched.

It starts at the end of the log rather than replaying the backlog, and
re-attaches in a loop: `qs log -f` ends when the instance it attached to exits,
and the dev shell outlives individual instances.

Also corrects the hot-reload note added in 7268221, which was wrong on both
counts. Tested against the running shell: `touch` never reloads (mtime is not a
content change) and inode-replacing edits like `sed -i` are picked up fine. The
reliable check is whether `qs log` shows a "Reloading configuration..." line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HEtXvseVb5gwAtKNhFx2PU

flake.nix

@@ -583,6 +583,7 @@
           # instance is the identical build to the packaged one.
           qs = "${nixpkgs.legacyPackages.${system}.quickshell}/bin/qs";
           git = "${nixpkgs.legacyPackages.${system}.git}/bin/git";
+          grep = "${nixpkgs.legacyPackages.${system}.gnugrep}/bin/grep";
 
           # Resolved at RUN time, not build time: the entire point is to run the
           # working tree, and `self` here is only a store snapshot of it.
@@ -631,6 +632,33 @@
             echo "qs-dev: live on $cfg — edits there now hot-reload"
           '';
 
+          # qs log -f prints everything the instance logs; WARN and ERROR are the
+          # two that mean something is wrong with the QML in front of you. A
+          # binding loop or a failed binding is a WARN and easy to miss when it
+          # scrolls past inside a reload's worth of chatter.
+          qs-log = pkgs.writeShellScriptBin "qs-log" ''
+            set -uo pipefail
+            ${preamble}
+
+            filter='WARN|ERROR'
+            case "''${1:-}" in
+              -a|--all) filter='.' ;;
+            esac
+
+            # -t 1: `qs log -f` replays the whole backlog first, which would dump
+            # every historical warning into the terminal on shell entry.
+            #
+            # `qs log -f` ends when the instance it attached to exits, and the dev
+            # shell outlives individual instances — a QML error kills one, `qs-dev`
+            # starts another. Re-attach instead of going quiet for the session.
+            while :; do
+              if running "$cfg"; then
+                ${qs} log -p "$cfg" -t 1 -f 2>/dev/null | ${grep} --line-buffered -E "$filter" >&2
+              fi
+              sleep 1
+            done
+          '';
+
           qs-prod = pkgs.writeShellScriptBin "qs-prod" ''
             set -uo pipefail
             ${preamble}
@@ -641,7 +669,7 @@
           '';
         in
         pkgs.mkShell {
-          packages = [ pkgs.quickshell qs-dev qs-prod ];
+          packages = [ pkgs.quickshell qs-dev qs-prod qs-log ];
 
           # Swap on entry, swap back on exit. Three guards:
           #   - interactive only ($- has i). `nix develop --command X` EXECs X,
@@ -655,9 +683,15 @@
           shellHook = ''
             if [[ $- == *i* ]] && [ -n "''${WAYLAND_DISPLAY:-}" ] && [ -z "''${HOMELAB_QS_DEV:-}" ]; then
               export HOMELAB_QS_DEV=1
-              qs-dev && trap qs-prod EXIT
+              if qs-dev; then
+                # Stream the dev instance's warnings and errors into this
+                # terminal, and take the follower down with the shell.
+                qs-log & HOMELAB_QS_LOG=$!
+                trap 'kill "$HOMELAB_QS_LOG" 2>/dev/null; qs-prod' EXIT
+              fi
             fi
             echo "homelab devshell — qs-dev (working tree) / qs-prod (packaged); exit restores"
+            echo "homelab devshell — quickshell WARN/ERROR stream here; qs-log -a for everything"
           '';
         };
     };