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_notes.py

@@ -0,0 +1,56 @@
+"""Tests for notes 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_notes_calls_get(client):
+    from fable_mcp.tools.notes import list_notes
+    client.get = AsyncMock(return_value={"notes": [], "total": 0})
+    result = await list_notes(client, limit=10)
+    client.get.assert_called_once()
+    call_args = client.get.call_args
+    assert "/api/notes" in call_args[0][0]
+
+
+@pytest.mark.asyncio
+async def test_get_note_calls_correct_endpoint(client):
+    from fable_mcp.tools.notes import get_note
+    client.get = AsyncMock(return_value={"id": 42, "title": "Hello"})
+    result = await get_note(client, note_id=42)
+    client.get.assert_called_once_with("/api/notes/42")
+    assert result["id"] == 42
+
+
+@pytest.mark.asyncio
+async def test_create_note_posts_payload(client):
+    from fable_mcp.tools.notes import create_note
+    client.post = AsyncMock(return_value={"id": 1, "title": "My Note"})
+    result = await create_note(client, title="My Note", body="Content")
+    client.post.assert_called_once()
+    call_kwargs = client.post.call_args[1]
+    assert call_kwargs["json"]["title"] == "My Note"
+    assert call_kwargs["json"]["body"] == "Content"
+
+
+@pytest.mark.asyncio
+async def test_update_note_patches_payload(client):
+    from fable_mcp.tools.notes import update_note
+    client.patch = AsyncMock(return_value={"id": 1, "title": "Updated"})
+    result = await update_note(client, note_id=1, title="Updated")
+    client.patch.assert_called_once()
+    path = client.patch.call_args[0][0]
+    assert "/api/notes/1" in path
+
+
+@pytest.mark.asyncio
+async def test_delete_note_calls_delete(client):
+    from fable_mcp.tools.notes import delete_note
+    client.delete = AsyncMock(return_value=None)
+    await delete_note(client, note_id=5)
+    client.delete.assert_called_once_with("/api/notes/5")