feat: add BranchAwarePromptBuilder with unit tests

Pure function that assembles system prompt, cross-branch context,
history, and images for _call_ai — no DB access, no LLM calls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/branch_aware_prompt_builder.py

@@ -0,0 +1,58 @@
+"""Branch-aware prompt builder — assembles AI context with cross-branch awareness.
+
+Pure function: takes data, returns dict matching _call_ai parameter names.
+No DB access, no LLM calls. The caller pre-fetches all data.
+
+Return keys: system_base, rag_context, history, new_message, images
+- system_base: stable system prompt (cached by Anthropic)
+- rag_context: cross-branch summaries + attachment descriptions (NOT cached)
+"""
+from typing import Any
+
+from app.services.assistant_chat_service import ASSISTANT_SYSTEM_PROMPT
+
+
+class BranchAwarePromptBuilder:
+    """Assembles prompt components for branch-aware AI calls."""
+
+    def build(
+        self,
+        branch_messages: list[dict[str, Any]],
+        sibling_summaries: str,
+        session_context: str,
+        attachments: list[dict[str, Any]],
+        new_message: str,
+        revival_context: str | None = None,
+        token_budget: int = 100_000,
+    ) -> dict[str, Any]:
+        """Build prompt components for _call_ai.
+
+        Returns dict with keys: system_base, rag_context, history, new_message, images.
+        """
+        # 1. system_base — stable, cached across turns
+        system_base = ASSISTANT_SYSTEM_PROMPT + "\n\n## Session Context\n" + session_context
+
+        # 2. rag_context — changes per query, NOT cached
+        rag_parts = []
+        if revival_context:
+            rag_parts.append(f"\n## Branch Revival\n{revival_context}")
+        if sibling_summaries:
+            rag_parts.append(sibling_summaries)
+        rag_context = "\n".join(rag_parts)
+
+        # 3. history — branch messages filtered to user/assistant
+        history = []
+        for msg in branch_messages:
+            if msg.get("role") in ("user", "assistant"):
+                history.append({"role": msg["role"], "content": msg["content"]})
+
+        # 4. images
+        images = attachments if attachments else None
+
+        return {
+            "system_base": system_base,
+            "rag_context": rag_context,
+            "history": history,
+            "new_message": new_message,
+            "images": images,
+        }