feat(fable-mcp): implement FableClient, all tool modules, and server.py

- client.py: async httpx wrapper, FableAPIError, stream_get() SSE generator,
  singleton init_client()/get_client() and _reset_client() for tests
- tools/: notes, tasks, projects, milestones, search, chat — thin async
  functions that accept FableClient and call Fable REST endpoints
- server.py: FastMCP entry point with 20 tools registered via @mcp.tool(),
  each opening a fresh FableClient context per call; validates env vars at startup
- tests: 34 tests covering client HTTP behaviour, error handling, singleton,
  SSE streaming, and all tool modules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fable-mcp/tests/test_tools_milestones.py

@@ -0,0 +1,35 @@
+"""Tests for milestones MCP tools."""
+import pytest
+from unittest.mock import AsyncMock
+
+
+@pytest.fixture
+def client(mock_client):
+    return mock_client
+
+
+@pytest.mark.asyncio
+async def test_list_milestones_calls_correct_endpoint(client):
+    from fable_mcp.tools.milestones import list_milestones
+    client.get = AsyncMock(return_value={"milestones": []})
+    await list_milestones(client, project_id=3)
+    client.get.assert_called_once_with("/api/projects/3/milestones")
+
+
+@pytest.mark.asyncio
+async def test_create_milestone_posts_to_project_endpoint(client):
+    from fable_mcp.tools.milestones import create_milestone
+    client.post = AsyncMock(return_value={"id": 10, "title": "M1"})
+    await create_milestone(client, project_id=3, title="M1")
+    path = client.post.call_args[0][0]
+    assert path == "/api/projects/3/milestones"
+    assert client.post.call_args[1]["json"]["title"] == "M1"
+
+
+@pytest.mark.asyncio
+async def test_update_milestone_patches(client):
+    from fable_mcp.tools.milestones import update_milestone
+    client.patch = AsyncMock(return_value={"id": 10, "status": "completed"})
+    await update_milestone(client, project_id=3, milestone_id=10, status="completed")
+    path = client.patch.call_args[0][0]
+    assert "/api/projects/3/milestones/10" in path