docs: add Javadoc to secrets package interfaces

[ginccc]

Summary

The secrets package implements the Secrets Vault (envelope encryption for API keys, tokens, and other sensitive configuration). Its 3 public interfaces lack Javadoc.

Interfaces to document (3)

• src/main/java/ai/labs/eddi/secrets/ISecretProvider.java
• src/main/java/ai/labs/eddi/secrets/persistence/ISecretPersistence.java
• src/main/java/ai/labs/eddi/secrets/rest/IRestSecretStore.java

How to write good Javadoc

1. Read the implementation class to understand what the interface does
2. Look at the model class it operates on
3. Write 2-3 lines: purpose + what consumes it + key behaviors

Good example (see IConversationMemoryStore):

/**
 * Persistence store for conversation memory snapshots.
 * Called by the lifecycle pipeline to save/load conversation state
 * between turns and across server restarts.
 */

Bad example (don't do this):

/** Store for agents. */

Note: If the interface currently has only /** @author ginccc */, replace it with
meaningful Javadoc. Per CONTRIBUTING.md, @author tags are not used in new code.

Acceptance criteria

• All 3 interfaces have class-level Javadoc
• Any existing @author tags are removed
• ./mvnw compile passes

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.

[nightcityblade]

Quick update: I checked the current main branch and these three interfaces already have class-level Javadoc, so I won't take this one after all.