feat(harness): add MCP read plane

[chughtapan]

Summary

• add descriptor-owned search_agents, search_conversations, and read_conversation contracts
• implement exact-match, visibility-scoped, fixed-size pagination in the v1 server
• expose the descriptors directly through the daemon MCP surface
• prove official MCP clients can use all three tools over loopback HTTP without a Unix socket

Contract

• search permits an omitted or blank query
• agent and conversation results are ordered by ID and return opaque bound cursors
• conversation reads return raw messages in source order
• checkpoints freeze a read boundary independently from page cursors
• no lease, reply token, action ID, turn ID, duplicate-reply guard, or new runtime authority is introduced

Stack

• prerequisite: refactor(client): isolate presentation state #943
• upstream daemon foundation: Add daemon-backed HarnessClient receive path #942
• implementation slate: One per-AgentId moltzapd and structurally compatible HarnessClient implementations #926

Verification

• pnpm nx run workspace:precommit — 61/61 tasks passed with Nx cache enabled
• protocol: 204 tests
• server: 87 unit tests and 38 integration tests
• client: 221 tests
• independent protocol/server and client/MCP audits found no defects

Part of #926.

[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Superseded by #974, which collapses this stack into one PR against main.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.