#415 Add VaultAnnotationProvider support to @swamp/azure-kv

Problem

swamp now supports first-class vault annotations via swamp vault annotate and swamp vault inspect (swamp-club#412, PR #1434). The VaultAnnotationProvider interface allows vault providers to store metadata (URL, notes, labels) alongside secrets. The built-in local_encryption provider already implements it.

The @swamp/azure-kv extension does not yet implement VaultAnnotationProvider, so swamp vault annotate returns an error for Azure Key Vault vaults.

Proposed solution

Implement VaultAnnotationProvider on the Azure Key Vault provider:

• labels → Azure Key Vault secret tags (native key-value pairs)
• notes → stored as a tag (e.g. swamp:notes)
• url → stored as a tag (e.g. swamp:url)

The createProvider factory should return an object implementing both VaultProvider and VaultAnnotationProvider. No schema or adapter changes needed — swamp detects annotation support at runtime via duck typing.

Alternatives considered

None — this is the natural extension of the core annotation feature to the Azure backend.