feat(sprint5): quality pass, smoke test workflow, Get-CIJobSummary

Task 5.1: Delete runner/Install-Runner.ps1 (superseded by Setup-Host.ps1).
Task 5.2: Replace Unicode/Discord emoji in Watch-DiskSpace.ps1, Watch-RunnerHealth.ps1,
          Test-E2E-Section3.3.ps1 with plain-text prefixes ([WARNING], [ALERT], [WARN]).
Task 5.3: docs/TEST-PLAN-v1.3-to-HEAD.md: ForEach-Object -Parallel replaced with Start-Job;
          -RepositoryUrl replaced with -RepoUrl (3 occurrences); PS7 syntax removed.
Task 5.4: scripts/Get-CIJobSummary.ps1 (new): scans JSONL logs in F:\CI\Logs, prints summary
          table per job, -JobId detail view, -Failed filter, -Last N.
Task 3.5: gitea/workflows/self-test.yml (new): workflow_dispatch, smoke-windows + smoke-linux.
          scripts/Test-Smoke.ps1 (new): invokes Invoke-CIJob.ps1 with marker-file build,
          asserts artifact dir and JSONL success event.
Task 5.6: docs/BEST-PRACTICES.md Section 10 added: SHA256 pinning policy, priority table,
          PS 5.1 implementation pattern, update guidance.

scripts/Watch-DiskSpace.ps1

@@ -83,7 +83,7 @@ try {
 
 # Optional webhook (Discord / Gitea)
 if ($WebhookUrl) {
-    $body = @{ content = ":warning: **CI Disk Alert** — $msg" } | ConvertTo-Json -Compress
+    $body = @{ content = "[WARNING] CI Disk Alert -- $msg" } | ConvertTo-Json -Compress
     try {
         $null = Invoke-RestMethod -Uri $WebhookUrl -Method Post `
             -Body $body -ContentType 'application/json' -TimeoutSec 10