Mark .pkg format for sunset, remove from README

Add TODO(pkg-sunset) tags at .pkg code entry points so a future bulk
removal can grep -r pkg-sunset. Remove .pkg from README examples and
format lists.

Add universal source structure section, clarify MSI scope

Add "Package Source Structure" near the top showing the one source
layout that applies to all output formats. Clarify that sbin-installer
is tuned for cimipkg-built MSIs and third-party MSIs are better
served by msiexec directly.

Update README for MSI-first world and pkg sunset

Document MSI as the default and recommended package format. Add MSI
section covering DTF-based installation, embedded build-info.yaml via
CIMIAN_PKG_BUILD_INFO, VBScript custom actions with pwsh 7 runtime
detection, deterministic UpgradeCode, PE file versioning for overwrite
semantics, and the conflict resolution / repair flow.

Update nupkg structure to show chocolateyBeforeModify.ps1 (replaces
the non-existent chocolateyBeforeInstall.ps1 that was listed before).
Note the Chocolatey limitation where BeforeModify only fires on
upgrade/uninstall, and sbin-installer's advantage of running it
unconditionally.

Mark .pkg as deprecated with a migration note pointing to MSI.

Update comparison table, project structure, build-info.yaml example
(with ${TIMESTAMP} and ${SIGNING_CERT_THUMBPRINT} placeholders), and
installation process flows for all three formats.

Merge pull request #4 from windowsadmins/fix/release-body

Add signing instructions to release body