feat(api): prompt loader (header split) + curated canon-log digest

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

api/app/prompts.py

@@ -0,0 +1,57 @@
+"""Load a role's system prompt (§16 — prompts are source code) and render the
+canon log into a curated digest for the user message (charter §11).
+
+The digest is a projection, not a dump: it surfaces narrative context and
+deliberately omits raw integers — companion dispositions render as names only,
+luck as its descriptor, and ids/schema_version are dropped (charter §7/§2).
+"""
+
+from functools import lru_cache
+from pathlib import Path
+
+_PROMPT_DIR = Path(__file__).resolve().parent.parent / "prompts"
+
+
+@lru_cache(maxsize=None)
+def system_prompt(role: str) -> str:
+    text = (_PROMPT_DIR / f"{role}.md").read_text(encoding="utf-8")
+    # Everything below the first '---' line is the model-facing prompt; the
+    # header above it is human/dev front-matter.
+    parts = text.split("\n---\n", 1)
+    body = parts[1] if len(parts) == 2 else text
+    return body.strip()
+
+
+def render_digest(canon_log: dict) -> str:
+    lines: list[str] = []
+
+    location = canon_log.get("location", {})
+    lines.append(f"Location: {location.get('name', 'an unknown place')}")
+
+    events = canon_log.get("recent_events", [])
+    if events:
+        lines.append("Recent events:")
+        lines += [f"- {e}" for e in events]
+
+    facts = canon_log.get("established_facts", [])
+    if facts:
+        lines.append("Established facts:")
+        lines += [f"- {f}" for f in facts]
+
+    party = canon_log.get("party", [])
+    if party:
+        lines.append("Companions present: " + ", ".join(m.get("name", "") for m in party))
+
+    for quest in canon_log.get("active_quests", []):
+        if quest.get("status") == "active":
+            lines.append(f"Active quest: {quest.get('name', '')} — {quest.get('objective', '')}")
+
+    player = canon_log.get("player", {})
+    descriptor = player.get("luck_descriptor")
+    if descriptor:
+        lines.append(f"Fortune: {descriptor}")
+    lines.append(f"Player: {player.get('name', '')}, a {player.get('class_id', '')}")
+
+    lines.append("")
+    lines.append("Describe the scene as it is now.")
+    return "\n".join(lines)