feat(providers): add ACP Client Protocol provider

[joka-7]

Summary

Adds an ACP Client Protocol provider that lets NanoClaw act as the
editor/client side of the Agent Client Protocol
(JSON-RPC 2.0 over TCP or stdio). Any external AI coding agent that speaks
ACP can be wired to an agent group — no container image rebuild required.

Protocol overview

ACP Client Protocol defines a JSON-RPC 2.0 exchange between an editor
(NanoClaw) and an agent (any ACP-compliant AI). The wire format is one
JSON object per line (\n-delimited), over either a TCP socket or stdin/stdout.

Handshake

editor → agent: initialize
← agent: { protocolVersion, agentCapabilities, meta, authMethods }

editor → agent: session/new { cwd, mcpServers }
← agent: { sessionId }

Per-message flow

editor → agent: session/prompt { sessionId, prompt: [{ type: "text", text }] }
← agent: session/update (notification, 0..N)
{ method: "session/update",
params: { sessionId, update: { sessionUpdate: "agent_message_chunk",
content: { role, content: [{ type, text }] } } } }
← agent: { content: [], stopReason: "end_turn" } (RPC response)

NanoClaw collects all agent_message_chunk text blocks, joins them, and delivers
the assembled message back through the originating channel.

File callbacks (optional)

If the agent declares agentCapabilities.fs.readTextFile / writeTextFile,
it may call back into NanoClaw mid-prompt:

agent → editor (RPC request): fs/read_text_file { path }
← editor: { content: "..." }

agent → editor (RPC request): fs/write_text_file { path, content }
← editor: {}

All paths are resolved under /workspace; requests outside that boundary
are rejected.

Session resume

sessionId is stored in outbound.db after each successful exchange.
On the next message NanoClaw sends the same sessionId in session/prompt.
If the agent returns a session-not-found error, NanoClaw automatically opens
a new session and retries transparently.

Connection modes

Mode	Config key	How it works
Subprocess	command	NanoClaw spawns the agent as a child process; JSON-RPC over stdin/stdout
TCP	host + port	NanoClaw connects to a running agent server

Files

File	Purpose
container/agent-runner/src/providers/acp-client.ts	Core provider: transport, dispatcher, session lifecycle
container/agent-runner/src/providers/acp-client.test.ts	33 unit tests (bun:test)
src/providers/acp-client.ts	Host-side config reader — injects env vars into the agent container
.claude/skills/add-acp-client-agent/SKILL.md	Install + wiring skill

Example configuration

Subprocess mode (groups/my-acp-agent/acp-client.json):

{ "command": ["python3", "/path/to/my-agent.py"] }

TCP mode:
{ "host": "host.docker.internal", "port": 7787 }

container.json (set "provider": "acp-client"):
{
  "provider": "acp-client",
  "groupName": "My ACP Agent",
  "assistantName": "My ACP Agent",
  "agentGroupId": "ag-..."
}

Testing

All 33 container tests pass:
cd container/agent-runner && bun test src/providers/acp-client.test.ts

A minimal echo-mode test server (scripts/test-acp-client-server.py) is
included for manual end-to-end verification — run it untracked alongside
a live NanoClaw instance.