Refactor: move compaction code to src/core/compaction/

- Move compaction.ts to src/core/compaction/compaction.ts
- Extract branch summarization to src/core/compaction/branch-summarization.ts
- Add index.ts to re-export all compaction utilities
- Update all imports across the codebase

packages/coding-agent/src/core/compaction/branch-summarization.ts

@@ -0,0 +1,115 @@
+/**
+ * Branch summarization for tree navigation.
+ *
+ * When navigating to a different point in the session tree, this generates
+ * a summary of the branch being left so context isn't lost.
+ */
+
+import type { Model } from "@mariozechner/pi-ai";
+import { complete } from "@mariozechner/pi-ai";
+import type { SessionEntry } from "../session-manager.js";
+
+const DEFAULT_INSTRUCTIONS =
+	"Summarize this conversation branch concisely, capturing key decisions, actions taken, and outcomes.";
+
+export interface BranchSummaryResult {
+	summary?: string;
+	aborted?: boolean;
+	error?: string;
+}
+
+/**
+ * Extract text content from any message type.
+ */
+function extractMessageText(message: any): string {
+	if (!message.content) return "";
+	if (typeof message.content === "string") return message.content;
+	if (Array.isArray(message.content)) {
+		return message.content
+			.filter((c: any) => c.type === "text")
+			.map((c: any) => c.text)
+			.join("");
+	}
+	return "";
+}
+
+/**
+ * Generate a summary of abandoned branch entries.
+ *
+ * @param entries - Session entries to summarize
+ * @param model - Model to use for summarization
+ * @param apiKey - API key for the model
+ * @param signal - Abort signal for cancellation
+ * @param customInstructions - Optional custom instructions for summarization
+ */
+export async function generateBranchSummary(
+	entries: SessionEntry[],
+	model: Model<any>,
+	apiKey: string,
+	signal: AbortSignal,
+	customInstructions?: string,
+): Promise<BranchSummaryResult> {
+	// Convert entries to messages for summarization
+	const messages: Array<{ role: string; content: string }> = [];
+
+	for (const entry of entries) {
+		if (entry.type === "message") {
+			const text = extractMessageText(entry.message);
+			if (text) {
+				messages.push({ role: entry.message.role, content: text });
+			}
+		} else if (entry.type === "custom_message") {
+			const text =
+				typeof entry.content === "string"
+					? entry.content
+					: entry.content
+							.filter((c): c is { type: "text"; text: string } => c.type === "text")
+							.map((c) => c.text)
+							.join("");
+			if (text) {
+				messages.push({ role: "user", content: text });
+			}
+		} else if (entry.type === "branch_summary") {
+			messages.push({ role: "system", content: `[Previous branch summary: ${entry.summary}]` });
+		}
+	}
+
+	if (messages.length === 0) {
+		return { summary: "No content to summarize" };
+	}
+
+	// Build prompt for summarization
+	const conversationText = messages.map((m) => `${m.role}: ${m.content}`).join("\n\n");
+	const instructions = customInstructions ? `${customInstructions}\n\n` : `${DEFAULT_INSTRUCTIONS}\n\n`;
+	const prompt = `${instructions}Conversation:\n${conversationText}`;
+
+	// Call LLM for summarization
+	const response = await complete(
+		model,
+		{
+			messages: [
+				{
+					role: "user",
+					content: [{ type: "text", text: prompt }],
+					timestamp: Date.now(),
+				},
+			],
+		},
+		{ apiKey, signal, maxTokens: 1024 },
+	);
+
+	// Check if aborted or errored
+	if (response.stopReason === "aborted") {
+		return { aborted: true };
+	}
+	if (response.stopReason === "error") {
+		return { error: response.errorMessage || "Summarization failed" };
+	}
+
+	const summary = response.content
+		.filter((c): c is { type: "text"; text: string } => c.type === "text")
+		.map((c) => c.text)
+		.join("\n");
+
+	return { summary: summary || "No summary generated" };
+}