Update Harn's ACP integration

This guide is for maintainers changing Harn's ACP server, ACP LLM provider, or Harn-owned ACP extensions. For the public API, see MCP, ACP, and A2A integration. For the current SDK decision, see ADR 0006.

Current boundary

Harn still owns the ACP runtime. Do not add agent-client-protocol as a production dependency until ADR 0006's revisit checklist is green, including an unmodified Zed session against an SDK-backed harn serve acp.

When that cutover lands, mirror the MCP ownership split in Update Harn's MCP integration:

SDK owns protocol mechanics: typed messages and capabilities, version negotiation, JSON-RPC framing, request association, and transport shutdown.
Harn owns product policy: VM conversion, session workspace anchors, modes, model pin, thought level, budget, forking policy, inject/remind, timeline, approvals, and audit events.

Until then, treat the official SDK schema as an oracle for drift checks, not as the runtime.

Owning paths

crates/harn-serve/src/adapters/acp/ for the serve adapter
crates/harn-vm/src/llm/providers/acp.rs for the ACP LLM provider
crates/harn-cli/src/acp/ and crates/harn-cli/src/commands/orchestrator/listener/acp_hub.rs for CLI and multi-client hub entry
docs/src/spec/harn-extensions/v1.md for Harn-owned extension fields
spec/acp-registry/ for registry submission artifacts

Extension rules

Prefer stable upstream ACP methods when they exist.
Prefer _-prefixed method names for new Harn-only RPC (ACP extensibility). Existing harn.* methods are compatibility surface; do not grow that prefix without a migration plan.
Keep _meta.harn.* product metadata on open meta maps. Do not fork upstream types only to rename fields.
Custom sessionUpdate kinds that are not in the upstream enum need an explicit untyped or _-prefixed escape hatch before any SDK typed path owns emission.

Prove changes

Start with the narrow ACP suite that covers the changed seam, then broaden:

make check-docs-snippets
make check-drift

Editor-host claims still need the manual smoke path in ACP editor hosts until an automated unmodified-client suite exists.