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/_Common.psm1

@@ -29,6 +29,8 @@ function New-CISessionOption {
     [System.Management.Automation.Remoting.PSSessionOption]
 #>
     [OutputType([System.Management.Automation.Remoting.PSSessionOption])]
+    [Diagnostics.CodeAnalysis.SuppressMessageAttribute('PSUseShouldProcessForStateChangingFunctions', '',
+        Justification = 'New-CISessionOption creates an in-memory PSSessionOption object; no system state is changed.')]
     [CmdletBinding()]
     param()
     New-PSSessionOption -SkipCACheck -SkipCNCheck -SkipRevocationCheck