Add support to ignore and decorate Custom API implementations

[beyro]

Summary

dvx plugin adopt was scaffolding [PluginStep] attributes for Custom API implementations, which aren't event plugins — their handlers run at stage 30 (Main Operation), so the generated attributes were invalid. This change makes adopt, sync, and register recognize and skip Custom API registrations, and adds a [CustomApi] marker attribute to make the intent explicit in source.

Changes

• adopt: StepImporter skips Custom API steps — detected by stage == 30, a Custom API message, or a plugin type referenced by customapi.plugintypeid (new SdkMetadata lookups) — and marks the implementing class with [CustomApi] instead of [PluginStep].
• sync / register: PluginDiscovery skips [CustomApi] classes silently; the marker takes precedence over any [PluginStep] on the same class.
• New attribute: [CustomApi] in dvx.PluginAttributes marks an IPlugin class as a Custom API implementation.
• Packaging: dvx.PluginAttributes 1.1.2 → 1.2.0; dvx CLI package id dvx → dvx.cli, 1.6.0 → 1.6.1.

Testing

Full suite green (230 tests); builds across net462/net471/net9.0 with no warnings. New tests cover discovery skip (including [CustomApi] + [PluginStep]), the three adopt detection signals, and idempotent marker writing. README and RegressionTests.md updated.

Fixes #3