fix(refresh): refuse to promote an empty capture over a good snapshot

ssh exiting 0 is not proof the capture is usable — the inspect script can emit
nothing and both refresh scripts would mv that over a good system-details.txt and
report 'ok (0 bytes)'. Every reader tests the snapshot with -s, so the writer was
producing an artifact its own readers call invalid: a guard whose test disagrees
with its writer's contract has quietly stopped guarding. Prompted by
brokkr-smithy-dev hitting the same shape from the other side (a -s test against a
sentinel written with touch, a precondition that could never pass).

- empty capture -> refused, previous snapshot kept, host counted as failed (exit 1)
- capture under 1/4 of the previous -> promoted but flagged, since a host can
  legitimately shed services and the script should not guess
- header + CLAUDE.md contract lines corrected to say what is actually guaranteed
- verified red (empty inspect -> FAIL, snapshot intact, rc=1) then green (real host
  -> ok 6727 bytes)

CLAUDE.md

@@ -261,7 +261,7 @@ scripts/refresh-server-info.sh ana-docker
 
 Fleet-wide runs require the literal `all` keyword — no-args prints help so you can't accidentally hit every host by forgetting a name.
 
-The script pipes `server_inspect.sh` over SSH via stdin (no scp, no remote cleanup) and writes each `servers/<host>/system-details.txt` atomically — a failed run never clobbers the previous snapshot. The inspect script itself is read-only.
+The script pipes `server_inspect.sh` over SSH via stdin (no scp, no remote cleanup) and writes each `servers/<host>/system-details.txt` atomically — a failed run never clobbers the previous snapshot, **and neither does a run that succeeds but captures nothing**: `ssh` exiting 0 is not proof of a usable capture, so an empty result is refused, the previous snapshot is kept, and the host is counted as failed (2026-09-09 — the readers all test the snapshot with `-s`, so the writer must not produce something they would call invalid). A capture that collapses to under a quarter of the previous one still promotes but is flagged, because a host really can shed services. The inspect script itself is read-only.
 
 Each server dir can hold an `ssh-target` file (one line, `<ip>` or `<user>@<ip>`) as a fallback for when the dir name doesn't resolve via DNS or `~/.ssh/config`. The script prefers whatever ssh would resolve normally and only consults the file when that fails.