deploy.sh no longer starts units a live session already runs

`sync` on a box with a live session on it was unsafe in two independent ways, found by
preflighting the run rather than performing it. Refs #44.

**`enable --now` would have armed a screen lock mid-session.** The unit loop started every
`systemd-user/` unit whose `ExecStart` resolves. `swayidle-lock.service` on blankpad is
`enabled`/`inactive` — nothing started it, no swayidle process exists, `autostart.kdl` does
not launch one — so one `deploy.sh` would have begun blanking and locking the monitors while
the operator was mid-sentence. `live-sync.service` is `linked`, driven by its timer, and
`--now` forces a heartbeat commit at an arbitrary moment. A unit is now started only when
systemd reports it as nothing-yet (`""`/`disabled`), which is precisely a fresh box; anything
already installed is merely `enable`d. The state has to come from the **printed state**:
`is-enabled` exits 1 for `linked` exactly as for `disabled`, so exit status cannot
distinguish "installed, timer-driven" from "never installed" — the first version of this fix
got that wrong and the test caught it.

**`--check` output was being read as more than it can mean.** It prints `RELINK` for every
path the `zz` tier carries, even when the live link already points at the override
(`rules.kdl`, `keybinds.kdl`, `misc.kdl`, `autostart.kdl`, `.zshrc` all did on blankpad),
because the walk visits the base package first and reports the intermediate owner change.
The sentence committed in #43 — RELINK noise is "the sound of a stale override waiting" — is
too strong and is corrected: the line proves a pending change only when `readlink` shows the
live target is not already the `zz-$HOST` file, and `actions=0` is the honest summary. The
stale-override hazard #43 removed was and is real; the signal for it is the target, not the line.

**The dconf dump carried six of its 24 keys from another box**, none of them preferences:
a Firefox file-chooser `last-folder-path` of `/home/dank/Downloads`, a nautilus
`window-state` section, and `picture-uri`/`picture-uri-dark` pointing at
`file:///home/dank/.config/background`. The last five showed no conflict because blankpad's
live tree *already* held those `/home/dank` values — an earlier deploy wrote them here — and
a foreign value that agrees with live is invisible to any diff while still being wrong. They
are dropped; `picture-options='zoom'` stays, because it is a preference. The live wallpaper
URIs are cleared on the box as part of landing this (niri draws the wallpaper, noctalia owns
it, nothing reads a dead path).

`tests/deploy.bats` grows by four, over a stub `systemctl` — the real one on this box is
exactly what must not be invoked from a test. Red before: an already-`enabled` unit and a
`linked` unit both get `--now`. Green after, including the two directions that keep the
change honest: a `disabled` unit is still `enable --now`'d so a fresh box comes up, and a
unit whose `ExecStart` is absent here is never touched at all.

Verification: `bats tests/` 70/70, `kb-lint.sh omp-docs` 0, `kb-coverage.py` 0. The dump now
reconciles against live state with **0 conflicts and no `/home/` values in 18 keys**, proven
by key-by-key comparison rather than `diff` — `dconf load` writes only the keys present, so a
diff overstates the damage one way and hides it the other.

tests/deploy.bats

@@ -23,6 +23,24 @@ setup() {
   export HOME="$TEST_TMP/home"
   mkdir -p "$HOME"
   export DEPLOY_REPO="$REPO" DEPLOY_HOST=box
+
+  # systemctl stub: records every call and answers is-enabled from a file. The real
+  # systemctl must never run here — on this box it would start a screen lock.
+  mkdir -p "$TEST_TMP/bin"
+  export STUB_LOG="$TEST_TMP/systemctl.log" UNIT_STATE="$TEST_TMP/unit-state"
+  : > "$STUB_LOG"
+  cat > "$TEST_TMP/bin/systemctl" <<'STUB'
+#!/usr/bin/env bash
+echo "systemctl $*" >> "$STUB_LOG"
+if [ "$2" = is-enabled ]; then
+  state="$(cat "$UNIT_STATE" 2>/dev/null || echo disabled)"
+  printf '%s\n' "$state"
+  case "$state" in disabled | "") exit 1 ;; *) exit 0 ;; esac
+fi
+exit 0
+STUB
+  chmod +x "$TEST_TMP/bin/systemctl"
+  export PATH="$TEST_TMP/bin:$PATH"
 }
 
 teardown() { rm -rf "$TEST_TMP"; }
@@ -32,6 +50,13 @@ repo_file() { mkdir -p "$(dirname "$REPO/dotfiles/$1")"; printf '%s\n' "$2" > "$
 # live_file <absolute path under $HOME> <content>
 live_file() { mkdir -p "$(dirname "$1")"; printf '%s\n' "$2" > "$1"; }
 
+# systemd_unit <name> <ExecStart> — a unit in the systemd-user package
+systemd_unit() {
+  mkdir -p "$REPO/dotfiles/systemd-user/.config/systemd/user"
+  printf '[Unit]\nDescription=%s\n[Service]\nExecStart=%s\n[Install]\nWantedBy=default.target\n' "$1" "$2" \
+    > "$REPO/dotfiles/systemd-user/.config/systemd/user/$1"
+}
+
 @test "generated theme output is never adopted into a symlink" {
   repo_file "ghostty/.config/ghostty/themes/noctalia" 'background = "#f43841"'
   live_file "$HOME/.config/ghostty/themes/noctalia" 'background = "#f43841"'
@@ -74,3 +99,40 @@ live_file() { mkdir -p "$(dirname "$1")"; printf '%s\n' "$2" > "$1"; }
   grep -q 'theme = "tokyo-night"' "$HOME/.config/ghostty/config.ghostty"
   [ ! -d "$REPO/.quarantine" ]
 }
+
+@test "a unit already enabled is enabled but not started" {
+  systemd_unit lock.service /bin/sh
+  printf 'enabled\n' > "$UNIT_STATE"
+  run "$REPO/scripts/deploy.sh"
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"not started"* ]]
+  grep -q 'systemctl --user enable lock.service' "$STUB_LOG"
+  ! grep -q -- '--now' "$STUB_LOG"
+}
+
+@test "a linked unit is not started either" {
+  # `is-enabled` exits 1 for a linked unit, exactly as it does for a disabled one, so the
+  # printed state is the signal — exit status cannot tell them apart.
+  systemd_unit timer-driven.service /bin/sh
+  printf 'linked\n' > "$UNIT_STATE"
+  run "$REPO/scripts/deploy.sh"
+  [ "$status" -eq 0 ]
+  ! grep -q -- '--now' "$STUB_LOG"
+}
+
+@test "a disabled unit is enabled and started, so a fresh box still comes up" {
+  systemd_unit fresh.service /bin/sh
+  printf 'disabled\n' > "$UNIT_STATE"
+  run "$REPO/scripts/deploy.sh"
+  [ "$status" -eq 0 ]
+  grep -q 'systemctl --user enable --now fresh.service' "$STUB_LOG"
+}
+
+@test "a unit whose ExecStart is absent here stays inert" {
+  systemd_unit otherbox.service /nonexistent/daemon
+  printf 'disabled\n' > "$UNIT_STATE"
+  run "$REPO/scripts/deploy.sh"
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"SKIP"*otherbox.service* ]]
+  ! grep -q enable "$STUB_LOG"
+}