feat(FN-1741): add memory compaction feature

- Add MemoryCompaction service in core for compacting agent memory stores
- Add POST /api/memory/compact route handler in dashboard server
- Add compactMemory frontend API wrapper in dashboard app
- Add Compact Memory button to SettingsModal UI
- Add comprehensive tests for the compaction service and routes

packages/core/src/memory-compaction.ts

@@ -0,0 +1,214 @@
+/**
+ * AI Memory Compaction Service
+ *
+ * Provides AI-powered memory compaction for project memory files.
+ * Uses an AI agent to distill memory content down to the most important
+ * architectural conventions, pitfalls, and decisions.
+ *
+ * Features:
+ * - Dynamic import of @fusion/engine for AI agent creation
+ * - Read-only tool access (prevents accidental memory modification during compaction)
+ * - Session disposal in finally block to prevent leaks
+ * - AiServiceError for AI-related failures
+ */
+
+// Dynamic import for @fusion/engine to avoid resolution issues in test environment
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type AgentResult = any;
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+let createKbAgent: any;
+
+// Initialize the import (this runs in actual server, mocked in tests)
+async function initEngine() {
+  if (!createKbAgent) {
+    try {
+      // Use dynamic import with variable to prevent static analysis
+      const engineModule = "@fusion/engine";
+      const engine = await import(/* @vite-ignore */ engineModule);
+      createKbAgent = engine.createKbAgent;
+    } catch {
+      // Allow failure in test environments - agent functionality will be stubbed
+      createKbAgent = undefined;
+    }
+  }
+}
+
+// Initialize on module load (will be awaited in actual usage)
+const engineReady = initEngine();
+
+// ── Constants ───────────────────────────────────────────────────────────────
+
+/** System prompt for memory compaction */
+export const COMPACT_MEMORY_SYSTEM_PROMPT = `You are a memory distillation assistant for a software development project.
+
+Your job is to compress the provided project memory markdown into a shorter version that preserves only the most important information.
+
+## Guidelines
+- Preserve only the most important architectural conventions and patterns
+- Preserve critical pitfalls and anti-patterns to avoid
+- Preserve significant decisions and their rationale
+- Remove redundant examples, outdated information, and trivial details
+- Maintain the markdown format and structure
+- Output ONLY the compacted markdown - no explanations or commentary
+- Be aggressive in trimming while keeping essential knowledge
+
+## What to KEEP:
+- Key architectural patterns and their rationale
+- Important conventions that agents must follow
+- Critical pitfalls and how to avoid them
+- Major project decisions and their context
+- Security-sensitive patterns
+
+## What to REMOVE:
+- Verbose examples that can be inferred
+- Minor implementation details
+- Outdated or superseded information
+- Repetitive explanations
+- Trivial gotchas that aren't critical
+
+Return only the compacted markdown content.`;
+
+/** Debug flag for AI operations */
+const DEBUG = process.env.FUSION_DEBUG_AI === "true";
+
+// ── Custom Errors ───────────────────────────────────────────────────────────
+
+export class AiServiceError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "AiServiceError";
+  }
+}
+
+// ── AI Integration ───────────────────────────────────────────────────────────
+
+/**
+ * Compact memory content using AI to distill it down to the most important insights.
+ *
+ * @param content - The current memory content to compact
+ * @param rootDir - Project root directory for AI agent context
+ * @param provider - Optional AI model provider (e.g., "anthropic")
+ * @param modelId - Optional AI model ID (e.g., "claude-sonnet-4-5")
+ * @returns The compacted memory content
+ * @throws AiServiceError if AI processing fails
+ */
+export async function compactMemoryWithAi(
+  content: string,
+  rootDir: string,
+  provider?: string,
+  modelId?: string
+): Promise<string> {
+  // Ensure engine is loaded before using createKbAgent
+  await engineReady;
+
+  if (!createKbAgent) {
+    if (DEBUG) console.log("[memory-compaction] AI engine not available");
+    throw new AiServiceError("AI engine not available");
+  }
+
+  const agentOptions: {
+    cwd: string;
+    systemPrompt: string;
+    tools: "readonly";
+    defaultProvider?: string;
+    defaultModelId?: string;
+  } = {
+    cwd: rootDir,
+    systemPrompt: COMPACT_MEMORY_SYSTEM_PROMPT,
+    tools: "readonly",
+  };
+
+  // Add model selection if both provider and modelId are provided
+  if (provider && modelId) {
+    agentOptions.defaultProvider = provider;
+    agentOptions.defaultModelId = modelId;
+  }
+
+  if (DEBUG) console.log("[memory-compaction] Creating agent session...");
+  const agentResult = await createKbAgent(agentOptions);
+
+  if (!agentResult?.session) {
+    if (DEBUG) console.log("[memory-compaction] Failed to initialize AI agent - no session");
+    throw new AiServiceError("Failed to initialize AI agent");
+  }
+
+  if (DEBUG) console.log("[memory-compaction] Agent session created, sending prompt...");
+
+  try {
+    // Send the memory content to the agent
+    await agentResult.session.prompt(content);
+
+    // Check for session errors (pi SDK stores errors in state.error, does not throw)
+    if (agentResult.session.state?.error) {
+      const errorMsg = agentResult.session.state.error;
+      if (DEBUG) console.log(`[memory-compaction] Session error: ${errorMsg}`);
+      throw new AiServiceError(`AI session error: ${errorMsg}`);
+    }
+
+    if (DEBUG) console.log("[memory-compaction] Prompt sent, extracting response from messages...");
+
+    // Get the response text from the agent's state
+    interface AgentMessage {
+      role: string;
+      content?: string | Array<{ type: string; text: string }>;
+    }
+
+    const messages: AgentMessage[] = agentResult.session.state?.messages ?? [];
+    const assistantMessages = messages.filter((m: AgentMessage) => m.role === "assistant");
+
+    if (DEBUG) {
+      console.log(`[memory-compaction] Total messages: ${messages.length}, Assistant messages: ${assistantMessages.length}`);
+    }
+
+    const lastMessage = assistantMessages.pop();
+
+    let compacted = "";
+    if (lastMessage?.content) {
+      // Handle both string and array content types
+      if (typeof lastMessage.content === "string") {
+        compacted = lastMessage.content.trim();
+      } else if (Array.isArray(lastMessage.content)) {
+        // Extract text from content blocks
+        compacted = lastMessage.content
+          .filter((c: { type: string; text: string }): c is { type: "text"; text: string } => c.type === "text")
+          .map((c: { type: string; text: string }) => c.text)
+          .join("")
+          .trim();
+      }
+    }
+
+    if (DEBUG) console.log(`[memory-compaction] Extracted compacted content length: ${compacted.length}`);
+
+    if (!compacted) {
+      if (DEBUG) console.log("[memory-compaction] AI returned empty response");
+      throw new AiServiceError("AI returned empty response");
+    }
+
+    if (DEBUG) console.log("[memory-compaction] Memory compaction successful");
+    return compacted;
+  } catch (err) {
+    if (err instanceof AiServiceError) {
+      throw err;
+    }
+    const message = err instanceof Error ? err.message : "AI processing failed";
+    if (DEBUG) console.log(`[memory-compaction] Unexpected error: ${message}`);
+    throw new AiServiceError(message);
+  } finally {
+    // Ensure session is disposed even on error
+    try {
+      agentResult.session.dispose?.();
+    } catch {
+      // Ignore disposal errors
+    }
+  }
+}
+
+// ── Test Helpers ───────────────────────────────────────────────────────────
+
+/**
+ * Reset all compaction state. Used for testing only.
+ * Currently a no-op since there are no caches, but available for future use.
+ */
+export function __resetCompactionState(): void {
+  // No-op: no caches to reset in current implementation
+}