Define queued message withdrawal API and client

[V1ctor2182]

First release stage for #17.

What changes

• Defines POST /v0/agents/:agentId/messages/:messageId/withdraw as an additive API operation for pending user messages.
• Specifies durable tombstones, idempotent retries, 404/409 outcomes, and the message.withdrawn event.
• Adds the typed HappyAgentClient method and request/response/event contracts.
• Removes withdrawn IDs from reducer pending state.

Release sequencing

This intentionally contains only the specification and client layer. Per the repository API release flow, @slopus/happy-agent-client must be merged and released before the follow-up core, daemon route, dependency, and integration changes are synced.

Expected next client release: 0.0.52.

Verification

• @slopus/happy-agent-client typecheck passes.
• 40 focused client/reducer tests pass.
• The locally held follow-up implementation passes core checks and a daemon restart black-box scenario against these client types.