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-hooks-example.test.ts

@@ -1,66 +0,0 @@
-/**
- * Verify the documentation example from hooks.md compiles and works.
- */
-
-import { describe, expect, it } from "vitest";
-import type { HookAPI, SessionBeforeCompactEvent, SessionCompactEvent } from "../src/core/hooks/index.js";
-
-describe("Documentation example", () => {
-	it("custom compaction example should type-check correctly", () => {
-		// This is the example from hooks.md - verify it compiles
-		const exampleHook = (pi: HookAPI) => {
-			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) => `- ${typeof m.content === "string" ? m.content.slice(0, 100) : "[complex]"}`)
-					.join("\n");
-
-				// Hooks return compaction content - SessionManager adds id/parentId
-				return {
-					compaction: {
-						summary: `User requests:\n${summary}`,
-						firstKeptEntryId,
-						tokensBefore,
-					},
-				};
-			});
-		};
-
-		// Just verify the function exists and is callable
-		expect(typeof exampleHook).toBe("function");
-	});
-
-	it("compact event should have correct fields", () => {
-		const checkCompactEvent = (pi: HookAPI) => {
-			pi.on("session_compact", async (event: SessionCompactEvent) => {
-				// These should all be accessible
-				const entry = event.compactionEntry;
-				const fromHook = event.fromHook;
-
-				expect(entry.type).toBe("compaction");
-				expect(typeof entry.summary).toBe("string");
-				expect(typeof entry.tokensBefore).toBe("number");
-				expect(typeof fromHook).toBe("boolean");
-			});
-		};
-
-		expect(typeof checkCompactEvent).toBe("function");
-	});
-});