feat(coding-agent): context compaction with /compact, /autocompact, and auto-trigger

- Add /compact command for manual context compaction with optional custom instructions
- Add /autocompact command to toggle automatic compaction
- Auto-trigger compaction when context usage exceeds threshold (contextWindow - reserveTokens)
- Add CompactionComponent for TUI display with collapsed/expanded states
- Add compaction events to HTML export with collapsible summary
- Refactor export-html.ts to eliminate duplication between session and streaming formats
- Use setTimeout to break out of agent event handler for safe async compaction
- Show compaction summary in TUI after compaction completes

fixes #92

packages/coding-agent/src/tui/compaction.ts

@@ -0,0 +1,54 @@
+import { Container, Markdown, Spacer, Text } from "@mariozechner/pi-tui";
+import { getMarkdownTheme, theme } from "../theme/theme.js";
+
+/**
+ * Component that renders a compaction indicator with collapsed/expanded state.
+ * Collapsed: shows "Context compacted from X tokens"
+ * Expanded: shows the full summary rendered as markdown (like a user message)
+ */
+export class CompactionComponent extends Container {
+	private expanded = false;
+	private tokensBefore: number;
+	private summary: string;
+
+	constructor(tokensBefore: number, summary: string) {
+		super();
+		this.tokensBefore = tokensBefore;
+		this.summary = summary;
+		this.updateDisplay();
+	}
+
+	setExpanded(expanded: boolean): void {
+		this.expanded = expanded;
+		this.updateDisplay();
+	}
+
+	private updateDisplay(): void {
+		this.clear();
+		this.addChild(new Spacer(1));
+
+		if (this.expanded) {
+			// Show header + summary as markdown (like user message)
+			const header = `**Context compacted from ${this.tokensBefore.toLocaleString()} tokens**\n\n`;
+			this.addChild(
+				new Markdown(header + this.summary, 1, 1, getMarkdownTheme(), {
+					bgColor: (text: string) => theme.bg("userMessageBg", text),
+					color: (text: string) => theme.fg("userMessageText", text),
+				}),
+			);
+		} else {
+			// Collapsed: just show the header line with user message styling
+			const isMac = process.platform === "darwin";
+			const shortcut = isMac ? "CMD+O" : "CTRL+O";
+			this.addChild(
+				new Text(
+					theme.fg("userMessageText", `--- Earlier messages compacted (${shortcut} to expand) ---`),
+					1,
+					1,
+					(text: string) => theme.bg("userMessageBg", text),
+				),
+			);
+		}
+		this.addChild(new Spacer(1));
+	}
+}