Add elapsed run time, per-account results CSV, and a default config path

Summaries now show elapsed wall-clock time since the run started, and every
summary (interim and final) overwrites a results.csv (Object ID, UPN,
persona/status) next to the audit log, so an operator has a plain export
without parsing NDJSON. ConfigPath also now defaults to
./config/persona-engine.json instead of requiring -ConfigPath every run.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Presentation/Write-PersonaSummary.ps1

@@ -27,6 +27,11 @@ function Write-PersonaSummary {
     .PARAMETER Mode
         Preview or Enforce, shown in the header so a screenshot of a summary is
         self-describing.
+
+    .PARAMETER StartedUtc
+        Run start timestamp. When supplied, the header shows elapsed wall-clock
+        time since the run began. Optional so callers that only care about counts
+        are not forced to thread a clock through.
     #>
     [CmdletBinding()]
     param(
@@ -37,14 +42,25 @@ function Write-PersonaSummary {
         [string] $SummaryType = 'Interim',
 
         [ValidateSet('Preview', 'Enforce')]
-        [string] $Mode = 'Preview'
+        [string] $Mode = 'Preview',
+
+        [Nullable[datetime]] $StartedUtc
     )
 
     $reconciled = Test-PersonaReconciliation -Counters $Counters
 
+    # $StartedUtc arrives here already unwrapped to a plain [datetime] - PowerShell
+    # collapses [Nullable[datetime]] to DateTime (or $null) at the call boundary, so
+    # a null check is used rather than .Value / .HasValue.
+    $elapsed = ($null -ne $StartedUtc) ? ('{0:hh\:mm\:ss}' -f ([DateTime]::UtcNow - $StartedUtc)) : $null
+
     Write-Host ''
     Write-Host ('=' * 100) -ForegroundColor DarkGray
-    Write-Host ("{0} summary - mode: {1} - processed: {2}" -f $SummaryType, $Mode, $Counters.Processed) -ForegroundColor Cyan
+    Write-Host (
+        $elapsed `
+            ? ("{0} summary - mode: {1} - elapsed: {2} - processed: {3}" -f $SummaryType, $Mode, $elapsed, $Counters.Processed) `
+            : ("{0} summary - mode: {1} - processed: {2}" -f $SummaryType, $Mode, $Counters.Processed)
+    ) -ForegroundColor Cyan
     Write-Host ('=' * 100) -ForegroundColor DarkGray
 
     Write-Host ('{0,-28} {1,-40} {2,-9} {3,10} {4,8}' -f 'Rule ID', 'Name', 'Priority', 'Enabled', 'Matches') -ForegroundColor DarkGray