PreInstall runner: surface installer log on EXE failures (LogFile field)

When the runner runs a Type:MSI install it injects /L*v <log> and tails
that log on failure to show what actually went wrong. Type:EXE installs
had no equivalent - if Setup-OpenText.cmd or any other EXE wrapper
failed, the installlog just showed "Exit code 1 - FAILED" with no clue
what happened inside.

Adds an optional LogFile field to JSON entries. When present on a
Type:EXE entry, the runner:
  - Logs "Installer log: <path>" before launching the installer
  - On failure, tails the last 30 lines of that file into the runner
    log (same pattern as the MSI verbose log scan)

Wired up on the OpenText entry to point at C:\Logs\PreInstall\Setup-
OpenText.log (which Setup-OpenText.ps1 already writes itself). Other
EXE entries can opt in by adding their own LogFile field.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

playbook/preinstall/preinstall.json

@@ -96,6 +96,7 @@
       "Installer": "opentext\\Setup-OpenText.cmd",
       "Type": "EXE",
       "InstallArgs": "",
+      "LogFile": "C:\\Logs\\PreInstall\\Setup-OpenText.log",
       "PCTypes": ["Standard"]
     },
     {