Add #[Tag] class-level attribute for auto-tagging

[philipobenito]

Summary

Add a #[Tag('name')] class-level attribute that automatically tags a service when it is registered or auto-wired.

#[Tag('handlers')]
class OrderHandler implements HandlerInterface {}

Context

Tags are currently a per-registration concept via addTag(). A class-level #[Tag] attribute would allow services to self-declare their tags, reducing boilerplate in bootstrap code.

The committee deferred this because it implies class scanning, which is a bigger design decision. Without a scanning mechanism, the attribute would only apply during auto-wiring via ReflectionContainer or explicit registration, similar to how #[Shared] works.

Implementation notes

• Class-level attribute (Attribute::TARGET_CLASS | Attribute::IS_REPEATABLE)
• ReflectionContainer would need to read the attribute and communicate the tag to the container (the delegate container pattern makes this non-trivial since ReflectionContainer does not register definitions)
• Compilation support via DefinitionAnalyser::ensureSynthesisedIfNeeded() where synthesised CompiledDefinition objects could include tags from the attribute
• Consider whether this requires a class-scanning mechanism or works within the existing lazy-resolution model

Target

6.1 minor release