Phase 14: LLM-grounded encounter-balancing tip

- src/lib/assistant/prompts.ts: balanceSuggestionSchema + buildBalancePrompt
  (system message leads with the grounding constraint; restricts choices to the
  provided compendium candidates).
- useEncounterAdvisor hook: builds context → picks system-correct candidates →
  asks the LLM (when configured) for a structured pick, validates + filters names
  to the candidate set, else falls back to the deterministic buildSuggestedEncounter.
  Apply adds the creatures transactionally via encountersRepo.mutate.
- EncounterTipCard in the combat tracker: leads with the detected difficulty
  tendency (or the current difficulty), offers a one-click grounded suggestion
  (AI or deterministic) with propose→confirm Apply.

prompts unit tests + e2e for both the deterministic apply flow and the AI path
(stubbed provider); the AI test also confirms candidate-grounding (only
shortlisted creatures survive).

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

src/lib/assistant/prompts.ts

@@ -0,0 +1,46 @@
+import { z } from 'zod';
+import type { CampaignContext, CreatureCandidate } from './context';
+
+export const balanceSuggestionSchema = z.object({
+  reasoning: z.string(),
+  add: z.array(z.object({
+    name: z.string(),
+    count: z.number().int().min(1).max(8),
+    ref: z.string().optional(),
+  })),
+  targetDifficulty: z.string(),
+});
+export type BalanceSuggestion = z.infer<typeof balanceSuggestionSchema>;
+
+function partyLine(ctx: CampaignContext): string {
+  return ctx.party.map((p) => `${p.name} (level ${p.level} ${p.className})`).join(', ') || 'no PCs recorded';
+}
+
+/**
+ * Prompt for rebalancing the current encounter. The system message leads with the
+ * grounding constraint and restricts choices to the provided candidate list.
+ */
+export function buildBalancePrompt(
+  ctx: CampaignContext,
+  current: { difficulty: string; targetDifficulty: string; candidates: CreatureCandidate[] },
+): { system: string; user: string } {
+  const system =
+    `${ctx.systemConstraint}\n\n` +
+    `You are an encounter-balancing assistant for a ${ctx.systemLabel} game master. ` +
+    `Recommend which creatures to ADD to the current encounter to bring it to roughly "${current.targetDifficulty}" difficulty. ` +
+    `You MUST choose only from the provided candidate creatures (by exact name). Do not invent creatures or use any from another system. ` +
+    `Prefer 1–3 distinct creatures with sensible counts. Keep the reasoning to one or two sentences.`;
+
+  const candidateList = current.candidates
+    .map((c) => `- ${c.name} (rating ${c.rating}${c.hp ? `, hp ${c.hp}` : ''}${c.ac ? `, ac ${c.ac}` : ''})`)
+    .join('\n');
+
+  const user =
+    `Party: ${partyLine(ctx)}.\n` +
+    `Current encounter difficulty: ${current.difficulty}.\n` +
+    `Target difficulty: ${current.targetDifficulty}.\n\n` +
+    `Candidate creatures (choose only from these):\n${candidateList}\n\n` +
+    `Respond with the creatures to add (name + count), a short reasoning, and the targetDifficulty.`;
+
+  return { system, user };
+}