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/fable_mcp/tools/milestones.py

@@ -0,0 +1,53 @@
+"""MCP tools for Fable milestones."""
+from __future__ import annotations
+
+from typing import Any
+
+from fable_mcp.client import FableClient
+
+
+async def list_milestones(client: FableClient, *, project_id: int) -> dict[str, Any]:
+    """List milestones for a project."""
+    return await client.get(f"/api/projects/{project_id}/milestones")
+
+
+async def create_milestone(
+    client: FableClient,
+    *,
+    project_id: int,
+    title: str,
+    description: str = "",
+    status: str = "active",
+) -> dict[str, Any]:
+    """Create a milestone within a project."""
+    payload: dict[str, Any] = {
+        "title": title,
+        "description": description,
+        "status": status,
+    }
+    return await client.post(f"/api/projects/{project_id}/milestones", json=payload)
+
+
+async def update_milestone(
+    client: FableClient,
+    *,
+    project_id: int,
+    milestone_id: int,
+    title: str | None = None,
+    description: str | None = None,
+    status: str | None = None,
+    order_index: int | None = None,
+) -> dict[str, Any]:
+    """Update an existing milestone."""
+    payload: dict[str, Any] = {}
+    if title is not None:
+        payload["title"] = title
+    if description is not None:
+        payload["description"] = description
+    if status is not None:
+        payload["status"] = status
+    if order_index is not None:
+        payload["order_index"] = order_index
+    return await client.patch(
+        f"/api/projects/{project_id}/milestones/{milestone_id}", json=payload
+    )