feat(fase0): synthetic reference domain (D4) + backend profile skeleton (D2)

Completes Fase 0 (skeleton & decision-lock):

- reference_domain.py + data/reference_projects.json: a synthetic
  "anleggskostnad" portfolio (3 fictional construction-cost projects with
  cost line items) as the framework's bundled reference input. Plain typed
  loader (frozen dataclasses); the JSON-Schema data-source *contract* (B5)
  is deliberately deferred to Fase 2.
- backends.py: Profile (azure|local) + ChatBackend Protocol seam +
  AzureFoundryBackend/LocalBackend stubs + get_backend() selector
  (fail-fast on unknown profile). Empty skeleton per D2 — create_chat_client
  raises NotImplementedError until live wiring in Fase 1. Return type is the
  MAF BaseChatClient (the common base of FoundryChatClient/OpenAIChatClient).

Quality gate green: ruff format + check, mypy (src) clean, 12 pytest passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9FyyENxebxVThjrn9et8C

src/portfolio_optimiser/backends.py

@@ -0,0 +1,84 @@
+"""Backend profiles (D2): the seam between the framework and a MAF chat client.
+
+Per research §14, a MAF agent binds to a *chat client* and the model is a
+parameter on that client — so model choice is per-client (and later per-agent
+via a role->deployment model-map, B12). A "backend profile" selects how models
+are served and produces the corresponding MAF chat client.
+
+This is an EMPTY skeleton for Fase 0: it defines the profile selector and the
+seam (`ChatBackend`), with stubs that raise ``NotImplementedError``. Live wiring
+lands in Fase 1, when the Group Chat maker-checker spike needs real clients.
+
+Cost-discipline (D6): the ``LOCAL`` profile (an OpenAI-compatible local endpoint
+such as Ollama/LM Studio, driven by the GA ``agent-framework-openai`` client) is
+the development default; the ``AZURE`` profile (``FoundryChatClient``) is reserved
+for targeted, minimal verification against the private Foundry tenant.
+"""
+
+from __future__ import annotations
+
+from enum import Enum
+from typing import Protocol, runtime_checkable
+
+from agent_framework import BaseChatClient
+
+
+class Profile(str, Enum):
+    """Model-serving backend profile (D2)."""
+
+    AZURE = "azure"  # Foundry / Azure OpenAI — full profile
+    LOCAL = "local"  # OpenAI-compatible local endpoint — fallback / dev default
+
+
+@runtime_checkable
+class ChatBackend(Protocol):
+    """The seam: a backend produces a MAF chat client for a given model.
+
+    Implementations are Fase 0 skeletons — ``create_chat_client`` raises
+    ``NotImplementedError`` until wired in Fase 1. ``model`` is the
+    deployment/model name (resolved from the role->deployment model-map during
+    onboarding, B12).
+    """
+
+    profile: Profile
+
+    def create_chat_client(self, *, model: str) -> BaseChatClient:
+        """Create a MAF chat client bound to ``model``. Wired in Fase 1."""
+        ...
+
+
+class AzureFoundryBackend:
+    """AZURE profile: ``FoundryChatClient`` against a Foundry project (U18).
+
+    Skeleton — Foundry wiring (``project_endpoint``, ``credential``) lands in Fase 1.
+    """
+
+    profile = Profile.AZURE
+
+    def create_chat_client(self, *, model: str) -> BaseChatClient:
+        raise NotImplementedError(
+            "AzureFoundryBackend is a Fase 0 skeleton; Foundry wiring lands in Fase 1."
+        )
+
+
+class LocalBackend:
+    """LOCAL profile: ``OpenAIChatClient`` against an OpenAI-compatible local
+    endpoint (Ollama/LM Studio). Development default per cost-discipline (D6).
+
+    Skeleton — local-endpoint wiring (``base_url``, ``model``) lands in Fase 1.
+    """
+
+    profile = Profile.LOCAL
+
+    def create_chat_client(self, *, model: str) -> BaseChatClient:
+        raise NotImplementedError(
+            "LocalBackend is a Fase 0 skeleton; local-endpoint wiring lands in Fase 1."
+        )
+
+
+def get_backend(profile: Profile | str) -> ChatBackend:
+    """Select a backend by profile. Fail-fast (``ValueError``) on unknown profile."""
+    profile = Profile(profile)  # validates: ValueError on unknown string
+    if profile is Profile.AZURE:
+        return AzureFoundryBackend()
+    return LocalBackend()