Part of #3896 phase 1.

Summary

Add the in-memory manifest registry for messaging. This should make typed channel manifests discoverable for later compiler/planner work while keeping phase 1 isolated from current production workflows.

Scope

• Add src/lib/messaging/manifest/registry.ts.
• Support register(manifest), get(channelId), list(), and listAvailable(ctx).
• Keep the registry in-memory and side-effect free.
• Preserve current platform support semantics when filtering available manifests by agent.

Acceptance Criteria

• Duplicate channel IDs are rejected.
• Registered manifests can be retrieved by channel ID.
• list() returns registered manifests in registration order.
• listAvailable(ctx) filters by manifest-supported agent.
• listAvailable(ctx) treats an empty or omitted platform support list as no platform filter, matching current behavior.
• Existing code continues to read src/lib/sandbox/channels.ts; this issue only adds the new registry path.