[kieronlanning]

… refresh the settings docs

• The C# 15 updated memory-safety rules (MemorySafetyRules) are off by default again: the updated model is an experimental compiler feature, so tier 2 is opt-in. Tier 1 and the PRCFG0001 validation are unchanged, and AllowUnsafeBlocks is still never set.
• DisableSourceLink=true now opts out for real. Sdk.props re-assigned DisableSourceLink to false for every non-Roslyn project, clobbering a repository pre-import value, so the documented opt-out silently did nothing and the GitHub provider was always added.
• SourceLink is documented end to end: a wiki page, a README section and skill coverage explain the provider for each host (Azure Repos, Azure DevOps Server, GitLab, Bitbucket), the SourceLinkAzureDevOpsServerGitHost item, the CPM PackageVersion requirement and the CI detection. Microsoft.SourceLink.AzureDevOps.Git, which does not exist, is gone.
• Required settings and versions, how to set the target framework(s), and the compiler-visible (build_property.*) properties are now consistent across the README, the wiki and the skills.
• A repository-local skill, purview-buildsdk-settings-docs-sync, is tracked under .agents/skills with a .gitignore carve-out; InstalledSkillsAreNotTracked still fails if a skill the CLI installed is committed.
• SourceLinkTests and Dotnet11FeaturesTests pin the new behaviour.