feat: add ACP (Agent Client Protocol) server for editor integration

Adds full ACP support enabling hermes-agent to work as a coding agent
inside VS Code (via vscode-acp extension), Zed, JetBrains IDEs, and
any ACP-compatible editor.

## New module: acp_adapter/

- server.py: HermesACPAgent implementing all 15 Agent protocol methods
  (initialize, authenticate, new/load/list/fork/resume session, prompt,
  cancel, set mode/model/config, on_connect)
- session.py: Thread-safe SessionManager with per-session AIAgent lifecycle
- events.py: Callback factories translating hermes callbacks to ACP
  session_update notifications (tool_call, agent_thought, agent_message)
- tools.py: Tool kind mapping (20+ tools → read/edit/execute/search/fetch/think)
  and content builders (diffs for file edits, terminal output, text previews)
- permissions.py: Bridges hermes approval_callback to ACP requestPermission
  RPC for dangerous command approval dialogs in the editor
- auth.py: Provider credential verification
- entry.py: CLI entry point with .env loading and stderr logging

## Integration points

- run_agent.py: ACP tool bridge hook in _execute_tool_calls() for
  delegating file/terminal operations to the editor
- hermes_cli/main.py: 'hermes acp' subcommand
- pyproject.toml: [acp] optional dependency, hermes-acp entry point,
  included in [all] extras (auto-installed via install.sh)

## Supporting files

- acp_registry/agent.json: ACP Registry manifest
- acp_registry/icon.svg: Hermes caduceus icon
- docs/acp-setup.md: User-facing setup guide for VS Code, Zed, JetBrains

## Tests

- 41 new tests across 5 test files covering tools, sessions, permissions,
  server lifecycle, and auth
- Full test suite: 2901 passed, 0 failures

## User flow

1. hermes is already installed (install.sh)
2. Install 'ACP Client' extension in VS Code
3. Configure: command='hermes', args=['acp']
4. Chat with Hermes in the editor — diffs, terminals, approval
   dialogs, thinking blocks all rendered natively

tests/acp/test_server.py

@@ -0,0 +1,103 @@
+"""Tests for acp_adapter.server — HermesACPAgent ACP server."""
+
+import os
+from unittest.mock import MagicMock
+
+import pytest
+
+import acp
+from acp.schema import (
+    AgentCapabilities,
+    AuthenticateResponse,
+    Implementation,
+    InitializeResponse,
+    NewSessionResponse,
+    SessionInfo,
+)
+from acp_adapter.server import HermesACPAgent, HERMES_VERSION
+from acp_adapter.session import SessionManager
+
+
+@pytest.fixture()
+def mock_manager():
+    """SessionManager with a mock agent factory."""
+    return SessionManager(agent_factory=lambda: MagicMock(name="MockAIAgent"))
+
+
+@pytest.fixture()
+def agent(mock_manager):
+    """HermesACPAgent backed by a mock session manager."""
+    return HermesACPAgent(session_manager=mock_manager)
+
+
+# ---------------------------------------------------------------------------
+# initialize
+# ---------------------------------------------------------------------------
+
+
+class TestInitialize:
+    def test_initialize_returns_correct_protocol_version(self, agent):
+        resp = agent.initialize(protocol_version=1)
+        assert isinstance(resp, InitializeResponse)
+        assert resp.protocol_version == acp.PROTOCOL_VERSION
+
+    def test_initialize_returns_agent_info(self, agent):
+        resp = agent.initialize(protocol_version=1)
+        assert resp.agent_info is not None
+        assert isinstance(resp.agent_info, Implementation)
+        assert resp.agent_info.name == "hermes-agent"
+        assert resp.agent_info.version == HERMES_VERSION
+
+    def test_initialize_returns_capabilities(self, agent):
+        resp = agent.initialize(protocol_version=1)
+        caps = resp.agent_capabilities
+        assert isinstance(caps, AgentCapabilities)
+        assert caps.session_capabilities is not None
+        assert caps.session_capabilities.fork is not None
+        assert caps.session_capabilities.list is not None
+
+
+# ---------------------------------------------------------------------------
+# authenticate
+# ---------------------------------------------------------------------------
+
+
+class TestAuthenticate:
+    def test_authenticate_with_provider_configured(self, agent, monkeypatch):
+        monkeypatch.setenv("OPENROUTER_API_KEY", "sk-test-123")
+        resp = agent.authenticate(method_id="openrouter")
+        assert isinstance(resp, AuthenticateResponse)
+
+    def test_authenticate_without_provider(self, agent, monkeypatch):
+        monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
+        monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
+        monkeypatch.delenv("OPENAI_API_KEY", raising=False)
+        resp = agent.authenticate(method_id="openrouter")
+        assert resp is None
+
+
+# ---------------------------------------------------------------------------
+# new_session / cancel
+# ---------------------------------------------------------------------------
+
+
+class TestSessionOps:
+    def test_new_session_creates_session(self, agent):
+        resp = agent.new_session(cwd="/home/user/project")
+        assert isinstance(resp, NewSessionResponse)
+        assert resp.session_id
+        # Session should be retrievable from the manager
+        state = agent.session_manager.get_session(resp.session_id)
+        assert state is not None
+        assert state.cwd == "/home/user/project"
+
+    def test_cancel_sets_event(self, agent):
+        resp = agent.new_session(cwd=".")
+        state = agent.session_manager.get_session(resp.session_id)
+        assert not state.cancel_event.is_set()
+        agent.cancel(session_id=resp.session_id)
+        assert state.cancel_event.is_set()
+
+    def test_cancel_nonexistent_session_is_noop(self, agent):
+        # Should not raise
+        agent.cancel(session_id="does-not-exist")