Expand systemd specifiers before calling a unit foreign

The first real `deploy.sh` run on blankpad printed:

    SKIP   live-sync.service (ExecStart '%h/.omp/scripts/live-sync.sh' absent on this host)

while `/home/blank/.omp/scripts/live-sync.sh` sits right there. `%h` is expanded by systemd,
never by the shell, so the literal string was handed to `command -v`, failed, and the unit was
declared to belong to another machine — through the same code path whose whole purpose is to
keep a *foreign* unit inert, and silently. The victim was this repo's own heartbeat, and the
verdict was the same on every box that ever deployed it, because `%h` is exactly where the
checkout lives.

Nothing broke tonight only by luck of install state: `live-sync.service` is `linked`, driven
by its timer, so the heartbeat kept running while the deploy lied about it. A unit whose only
install route was this loop would simply never have been enabled anywhere.

`%h`, `%u` and `%U` are now expanded before resolution; an unrecognised specifier is left
alone so it fails loudly rather than being guessed at. Pinned by a test that writes a
`%h/.local/bin/helper` unit into the scratch home — red before (`SKIP`), green after
(`already linked; not started`), which also keeps the #45 rule intact: resolving a unit is not
licence to start it. Refs #44.

Verification: `bats tests/` 71/71, `kb-lint.sh omp-docs` 0 violations, `kb-coverage.py` 0
issues, `bash -n` clean.

omp-docs/tools/dotfiles-deploy.md

@@ -158,6 +158,27 @@ fresh box — where everything reports `disabled` — still gets its units up. B
 are pinned in `tests/deploy.bats` over a stub `systemctl`, because the real one on this box
 is exactly what must not be invoked from a test. Refs #44.
 
+## Law: expand systemd specifiers before calling a unit foreign
+
+`ExecStart=%h/.omp/scripts/live-sync.sh` is valid systemd and meaningless to a shell. The
+resolution check handed that literal string to `command -v`, which fails everywhere, so the
+deployment concluded the unit belonged to another machine and skipped it — silently, through
+the same code path that is supposed to keep a *foreign* unit inert. The unit it kept skipping
+was this repo's own heartbeat, and `%h` is precisely where this checkout lives, so the false
+verdict was the same on every box that deployed it. Measured on blankpad: the first real
+`deploy.sh` run printed `SKIP live-sync.service (ExecStart '%h/.omp/scripts/live-sync.sh'
+absent on this host)` while `/home/blank/.omp/scripts/live-sync.sh` sits right there.
+
+Expand the specifiers a user unit can carry — `%h`, `%u`, `%U` — before resolving, and let an
+unknown specifier fail rather than guess at it. The probe needs no privileges and writes
+nothing:
+
+```bash
+bin=$(sed -n 's/^ExecStart=\([^ ]*\).*/\1/p' \
+  ~/.omp/dotfiles/systemd-user/.config/systemd/user/live-sync.service | head -1)
+command -v "${bin//%h/$HOME}"   # resolves → the unit is this box's; the SKIP was the lie
+```
+
 ## Law: the dconf dump holds preferences, not per-box state
 
 `dconf load /` writes every key in `dotfiles/dconf/dump.txt`, so the dump carries whatever