Sprint 13-14: §6.2 composite action + §6.4 build matrix + §6.5 secret injection

§6.2 - New composite action gitea/actions/local-ci-build/action.yml
  Wraps Invoke-CIJob.ps1 for reuse across repos. Inputs: build-command,
  artifact-source, submodules, guest-os, use-git-clone, configuration,
  template-path, snapshot-name, artifact-name, artifact-retention-days.
  Anti-injection: all params via INPUT_* env vars, never interpolated in shell.

§6.4 - Build matrix windows+linux in build-nsis.yml
  gitea/workflows/build-nsis.yml rewritten with strategy.matrix target:
  [windows, linux], fail-fast: false, routes to {target}-build runner.
  Added action inputs: job-id-suffix (matrix disambig for artifact dirs),
  repo-url (SSH URL override for gitea-ci alias).

§6.5 - ExtraGuestEnv secret injection chain
  New param -ExtraGuestEnv [hashtable] in Invoke-CIJob.ps1 and
  Invoke-RemoteBuild.ps1. Windows: SetEnvironmentVariable in WinRM session
  ScriptBlock before build command. Linux: export KEY='val'; prefix before
  cd && buildcmd (POSIX single-quote escaping). Composite action input
  extra-guest-env-json (JSON object -> ConvertFrom-Json -> hashtable),
  never logged.

PSScriptAnalyzer fixes: _Common.psm1, Invoke-RetentionPolicy.ps1,
  Watch-RunnerHealth.ps1 (empty catch, ShouldProcess suppression, etc.)

Docs/test: TEST-PLAN-v1.3-to-HEAD.md updated §3.3/§6.1/§5.4 status.
  TODO.md: §6 fully closed (6.1-6.2 done, 6.3 deferred, 6.4-6.5 done, 6.6 done).

scripts/Watch-RunnerHealth.ps1

@@ -109,7 +109,7 @@ if (Test-Path $cooldownFile) {
     try {
         $raw = Get-Content $cooldownFile -Raw -ErrorAction SilentlyContinue
         if ($raw) { $restartLog = @($raw | ConvertFrom-Json) }
-    } catch { }
+    } catch { Write-Debug "[Watch-RunnerHealth] Restart log unreadable — starting fresh: $_" }
 }
 
 # Prune timestamps older than 1 hour