feat(harness): start conversations through MCP

[chughtapan]

Summary

• add the active start_conversation MCP tool with canonical other-agent names and required initial content
• resolve peers in moltzapd, create through the existing production conversation RPC, and persist the initial text through the existing message RPC
• return participant membership only in the loopback MCP projection; the canonical main WebSocket Conversation remains closed
• expose HarnessClient.startConversation while projecting its result back to the canonical Conversation shape

Production behavior retained

This is deliberately the minimal production composition selected for this track. Creation happens before the initial message send. If the send fails after creation, the conversation may already exist. This PR adds no atomicity, OperationId, retry recovery, size profile, or new partial-success/error contract.

Evidence

• @moltzap/client unit suite: 233 tests passed
• @moltzap/client integration target passed, including real server, direct WebSocket, loopback MCP start, persisted initial content, bound reply, and no Unix socket
• client lint and test typecheck passed
• full workspace precommit graph passed with normal Nx caching
• generated documentation drift and integrity checks passed
• independent code and authority reviews found no in-scope defect

Stack and provenance

• stacked on packaged moltzapd PR feat(client): package moltzapd process #947
• progresses implementation tracker One per-AgentId moltzapd and structurally compatible HarnessClient implementations #926
• upstream production control-plane simplification: refactor!: shrink the v1 control plane to a router #924

Upstream link: #924

[chughtapan]

Superseded by #974, which collapses this stack into one PR against main. This branch's commits are all contained in it — nothing is lost, and this thread stays readable. Closing to leave one place to review.