fix(prometheus): name the laptop cell runbook, not the UPS one (#454)

[Gerrrt]

HostBatteryHealthLow reads a laptop cell's measured full capacity against its design figure, and fires today for oracle's BAT0 at 72 %. Its description sent the reader to docs/runbooks/fit-the-ups-battery.md — the rack pack in mjolnir: an APCRBC115 in a Smart-UPS, a self-test to watch, an NMC to read. None of that applies to a thirteen-year-old cell in a laptop on a shelf, so the one alert whose entire action is "order the replacement" pointed at the wrong hardware.

It now names docs/runbooks/replace-the-laptop-cell.md, which carries the baseline, the fit, the mains-pull test, the stack-down window and the disposal. Because that runbook covers the before as well as the after, the sentence now reads "what to check before and after the swap" rather than only after.

⚠️ Merge order

That runbook lands with #497, which is still open. This PR should not merge ahead of it — otherwise the annotation names a path that does not yet exist on main. Nothing in validate.sh checks that a runbook path in rule prose resolves, so CI will not catch it; the ordering has to be held by hand.

Why the test moves too

The description is asserted verbatim in two exp_annotations blocks in tests/host.test.yaml, so rule and assertions had to change in the same commit. Line-wrapping differs between the two files and always has — the folded scalars were compared as strings to confirm they still produce identical text.

The other two rules in the block

HostOnBattery and HostBatteryNotReported were checked and deliberately left alone: neither names a runbook, so neither had a wrong one. Each already carries its own next step in prose — UpsOnBattery and node_power_supply_capacity for the first, /sys/class/power_supply/ for the second.

The fit-the-ups-battery.md reference in the block's header comment also stays. It cites that document for stating the shelf's mains-cut design, which is still true, not for how to change a cell.

No runbook_url annotation was added — this repo names runbooks as bare repo-relative paths inside the description prose, and that style is kept.

Verified

• promtool test rules on tests/host.test.yaml, via docker with the pinned prom/prometheus:v3.14.0 image — SUCCESS. (make check-rules invokes a bare promtool, which is not on PATH on this host.)
• Negative control: reverting one of the two assertions to the old path was confirmed to FAIL the run, proving the annotation is genuinely asserted and not silently ignored.
• ./scripts/validate.sh — exit 0, all checks passed, including observability: promtool test rules (11 file(s)). The 3 skips are the systemd unit checks that only run in the deployment checkout.
• Re-ran promtool test rules after rebasing onto the current main — still SUCCESS.

Refs #454

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