Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/test/compaction-extensions-example.test.ts

@@ -0,0 +1,66 @@
+/**
+ * Verify the documentation example from extensions.md compiles and works.
+ */
+
+import { describe, expect, it } from "vitest";
+import type { ExtensionAPI, SessionBeforeCompactEvent, SessionCompactEvent } from "../src/core/extensions/index.js";
+
+describe("Documentation example", () => {
+	it("custom compaction example should type-check correctly", () => {
+		// This is the example from extensions.md - verify it compiles
+		const exampleExtension = (pi: ExtensionAPI) => {
+			pi.on("session_before_compact", async (event: SessionBeforeCompactEvent, ctx) => {
+				// All these should be accessible on the event
+				const { preparation, branchEntries } = event;
+				// sessionManager, modelRegistry, and model come from ctx
+				const { sessionManager, modelRegistry } = ctx;
+				const { messagesToSummarize, turnPrefixMessages, tokensBefore, firstKeptEntryId, isSplitTurn } =
+					preparation;
+
+				// Verify types
+				expect(Array.isArray(messagesToSummarize)).toBe(true);
+				expect(Array.isArray(turnPrefixMessages)).toBe(true);
+				expect(typeof isSplitTurn).toBe("boolean");
+				expect(typeof tokensBefore).toBe("number");
+				expect(typeof sessionManager.getEntries).toBe("function");
+				expect(typeof modelRegistry.getApiKey).toBe("function");
+				expect(typeof firstKeptEntryId).toBe("string");
+				expect(Array.isArray(branchEntries)).toBe(true);
+
+				const summary = messagesToSummarize
+					.filter((m) => m.role === "user")
+					.map((m) => `- ${typeof m.content === "string" ? m.content.slice(0, 100) : "[complex]"}`)
+					.join("\n");
+
+				// Extensions return compaction content - SessionManager adds id/parentId
+				return {
+					compaction: {
+						summary: `User requests:\n${summary}`,
+						firstKeptEntryId,
+						tokensBefore,
+					},
+				};
+			});
+		};
+
+		// Just verify the function exists and is callable
+		expect(typeof exampleExtension).toBe("function");
+	});
+
+	it("compact event should have correct fields", () => {
+		const checkCompactEvent = (pi: ExtensionAPI) => {
+			pi.on("session_compact", async (event: SessionCompactEvent) => {
+				// These should all be accessible
+				const entry = event.compactionEntry;
+				const fromExtension = event.fromExtension;
+
+				expect(entry.type).toBe("compaction");
+				expect(typeof entry.summary).toBe("string");
+				expect(typeof entry.tokensBefore).toBe("number");
+				expect(typeof fromExtension).toBe("boolean");
+			});
+		};
+
+		expect(typeof checkCompactEvent).toBe("function");
+	});
+});