Add GuruRMM Agent MSI installer (WiX 5) — Phase 1 MVP

Signed Windows installer using our Azure Trusted Signing pipeline. Phase 1
scope: installs signed agent to Program Files, creates ProgramData dir,
Apps & Features entry with proper publisher, clean install + uninstall.

Phase 2 deferred: service registration, MSI properties for site-code
injection, agent install/uninstall custom actions, firewall rules.

Verified end-to-end on Windows workstation:
- wix build produces 1.16 MB MSI
- sign.ps1 signs it against gururmm-public-trust cert profile
- msiexec /qn installs silently, signature chain verifies on installed binary
- msiexec /x uninstalls cleanly, retains ProgramData

Tooling prerequisites documented in installer/README.md.

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

projects/msp-tools/guru-rmm/installer/.gitignore

@@ -0,0 +1,5 @@
+# Build artifacts — reproducible from wxs + downloaded binary
+*.msi
+*.wixpdb
+install-test.log
+src/gururmm-agent.exe