fix: lower tech-debt "no changes" threshold to 2 lines

The strong/weak split in #28 moved the commit URLs out of `report` and
into `reportFooter` (called after `prSummaryReport`), but the
`-le 5` threshold was kept. After that refactor the filtered report
only carries two boilerplate header lines, so any PR changing three or
fewer counters of a given level got silently reported as "no changes" —
e.g. leanprover-community/mathlib4#39932 removed
one `set_option backward.isDefEq.respectTransparency false in` without
the bot noting it.

Drop the threshold to 2 so a single non-zero change is reported.

🤖 Prepared with Claude Code

Author: kim-em

scripts/reporting/technical-debt-metrics.sh

@@ -191,7 +191,11 @@ prSummaryReport () {
   level="${1}"
   rep="${2}"
 
-  if [ "$(wc -l <<<"${rep}")" -le 5 ]
+  # After the `awk` filter in the caller, `rep` always contains the two boilerplate
+  # table-header lines (`|Current number|Change|Type|` and `|-:|:-:|:-|`).
+  # Any further line corresponds to a counter whose value changed, so a report
+  # with two or fewer lines means "nothing to report".
+  if [ "$(wc -l <<<"${rep}")" -le 2 ]
   then
     printf '<details><summary>No changes to %s technical debt.</summary>\n' "${level}"
     return 1