Resolve knowledge-index root to absolute path (cross-platform fix)

Get-ChildItem.FullName is always absolute, so deriving the relative article
path via Substring(\.Length) requires an absolute root. A relative root
such as '.' (used by the CI guard's 'Test-KnowledgeIndex.ps1 -Root .') left the
full path almost intact on Linux, producing bogus 'home/runner/.../knowledge'
paths and failing the coverage check. Normalise both the generator's
-BCQualityRoot and the test's -Root with Resolve-Path before use.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tools/Build-KnowledgeIndex.ps1

@@ -79,6 +79,10 @@ if (-not $BCQualityRoot) {
 if (-not (Test-Path $BCQualityRoot)) {
     throw "BCQuality root not found: $BCQualityRoot"
 }
+# Normalise to an absolute path: Get-ChildItem.FullName below is always
+# absolute, so Get-RelativePath's Substring needs an absolute root to strip.
+# A relative root (e.g. '.') would otherwise leave the full path intact.
+$BCQualityRoot = (Resolve-Path -LiteralPath $BCQualityRoot).Path
 if (-not $IndexPath) {
     $IndexPath = Join-Path $BCQualityRoot 'knowledge-index.json'
 }