Specification: UserMessages manifest field

Add specification for UserMessages manifest field (#3483). Includes:
- New UserMessages manifest field for displaying messages to users
- Flow behavior matrix for install, upgrade, uninstall, and download
- CLI arguments for UserMessages settings overrides
- Settings object for global UserMessages configuration
- PowerShell cmdlet integration details
- Deprecation path and schema impact guidance

Also updates spec-template.md with Deprecation Path section and
settings/arguments guidance.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: denelon

.github/actions/spelling/expect.txt

@@ -428,6 +428,7 @@ PEGI
 pfn
 pgp
 Pherson
+phishing
 pid
 pidl
 pidlist
@@ -586,6 +587,7 @@ TLSCAs
 tombstoned
 Toolhelp
 transitioning
+Trenly
 trimstart
 ttl
 twgc