docs(milestone): complete v0.2-ai-tutor-architecture

---ci---
phase: 7
milestone: v0.2
status: complete
requirements:
  covered: [REQ-2-001, REQ-2-002, REQ-2-003, REQ-2-004, REQ-2-005, REQ-2-006, REQ-2-007, REQ-2-008, REQ-2-009, REQ-2-010, REQ-2-011, REQ-2-012]
  partial: []
---/ci---

Milestone v0.2 (ai-tutor-architecture) merged to main.

Escalation record (audit remediation, durable): P1 executor
delegation failed twice (empty subagent results, zero files
created); auto-resolved at full autonomy to inline execution with
identical plan fidelity (commit 3271373, reflog-only after phase
branch squash-delete).

apps/ai-service/ai_service/llm/base.py

@@ -0,0 +1,35 @@
+"""LLMProvider protocol — the port all agents depend on (D-017).
+
+Implementations: openai_compat.OpenAICompatProvider (ollama-cloud + local),
+mock.MockProvider (deterministic, tests/CI). Providers are dumb pipes:
+no envelope logic here — the API layer owns meta/done/error events (D-016).
+"""
+
+from collections.abc import AsyncIterator
+from typing import Protocol
+
+from .types import Message
+
+
+class LLMProvider(Protocol):
+    async def stream_chat(
+        self,
+        messages: list[Message],
+        *,
+        model: str,
+        temperature: float = 0.7,
+        response_format: dict | None = None,
+    ) -> AsyncIterator[str]:
+        """Yield incremental content deltas (plain text chunks)."""
+        ...
+
+    async def chat(
+        self,
+        messages: list[Message],
+        *,
+        model: str,
+        temperature: float = 0.7,
+        response_format: dict | None = None,
+    ) -> str:
+        """Non-streaming completion — returns the full reply text."""
+        ...