Context

Issue #610 reports a Dependency-Check finding for com.github.bbottema:jetbrains-runtime-annotations:1.0.2 being mapped to JetBrains Runtime / CVE-2025-29903. The artifact used here is not JetBrains Runtime; it is a republished copy of JetBrains @NotNull / @Nullable with RUNTIME retention so the CLI generator can inspect @Nullable through reflection.

That design made the CLI clever but brittle: Java/API nullability and CLI argument optionality are currently coupled through the same annotation.

Goal

Separate those meanings explicitly:

• Keep official org.jetbrains:annotations for API nullability and standard IDE/static-analysis tooling.
• Remove com.github.bbottema:jetbrains-runtime-annotations from Simple Java Mail.
• Add explicit runtime-retained CLI metadata, probably @Cli.Optional or @Cli.OptionalParameter, for builder parameters that are optional in the CLI.
• Update CLI generation and command-line consumption to inspect that CLI annotation instead of org.jetbrains.annotations.Nullable.
• Preserve Java 8 compatibility.
• Document the builder/Javadoc/reflection/serialized-metadata CLI mechanism in PROJECT_MECHANISMS_CATALOGUE.md, including why the organization is brittle and how future API changes should keep CLI parity.

Notes

This should address the practical scanner/tooling concern from #610 without renaming the runtime fork or continuing to depend on the fork in Simple Java Mail.