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/examples/hooks/tools.ts

@@ -1,145 +0,0 @@
-/**
- * Tools Hook
- *
- * Provides a /tools command to enable/disable tools interactively.
- * Tool selection persists across session reloads and respects branch navigation.
- *
- * Usage:
- * 1. Copy this file to ~/.pi/agent/hooks/ or your project's .pi/hooks/
- * 2. Use /tools to open the tool selector
- */
-
-import { getSettingsListTheme } from "@mariozechner/pi-coding-agent";
-import type { HookAPI, HookContext } from "@mariozechner/pi-coding-agent/hooks";
-import { Container, type SettingItem, SettingsList } from "@mariozechner/pi-tui";
-
-// State persisted to session
-interface ToolsState {
-	enabledTools: string[];
-}
-
-export default function toolsHook(pi: HookAPI) {
-	// Track enabled tools
-	let enabledTools: Set<string> = new Set();
-	let allTools: string[] = [];
-
-	// Persist current state
-	function persistState() {
-		pi.appendEntry<ToolsState>("tools-config", {
-			enabledTools: Array.from(enabledTools),
-		});
-	}
-
-	// Apply current tool selection
-	function applyTools() {
-		pi.setActiveTools(Array.from(enabledTools));
-	}
-
-	// Find the last tools-config entry in the current branch
-	function restoreFromBranch(ctx: HookContext) {
-		allTools = pi.getAllTools();
-
-		// Get entries in current branch only
-		const branchEntries = ctx.sessionManager.getBranch();
-		let savedTools: string[] | undefined;
-
-		for (const entry of branchEntries) {
-			if (entry.type === "custom" && entry.customType === "tools-config") {
-				const data = entry.data as ToolsState | undefined;
-				if (data?.enabledTools) {
-					savedTools = data.enabledTools;
-				}
-			}
-		}
-
-		if (savedTools) {
-			// Restore saved tool selection (filter to only tools that still exist)
-			enabledTools = new Set(savedTools.filter((t: string) => allTools.includes(t)));
-			applyTools();
-		} else {
-			// No saved state - sync with currently active tools
-			enabledTools = new Set(pi.getActiveTools());
-		}
-	}
-
-	// Register /tools command
-	pi.registerCommand("tools", {
-		description: "Enable/disable tools",
-		handler: async (_args, ctx) => {
-			// Refresh tool list
-			allTools = pi.getAllTools();
-
-			await ctx.ui.custom((tui, theme, done) => {
-				// Build settings items for each tool
-				const items: SettingItem[] = allTools.map((tool) => ({
-					id: tool,
-					label: tool,
-					currentValue: enabledTools.has(tool) ? "enabled" : "disabled",
-					values: ["enabled", "disabled"],
-				}));
-
-				const container = new Container();
-				container.addChild(
-					new (class {
-						render(_width: number) {
-							return [theme.fg("accent", theme.bold("Tool Configuration")), ""];
-						}
-						invalidate() {}
-					})(),
-				);
-
-				const settingsList = new SettingsList(
-					items,
-					Math.min(items.length + 2, 15),
-					getSettingsListTheme(),
-					(id, newValue) => {
-						// Update enabled state and apply immediately
-						if (newValue === "enabled") {
-							enabledTools.add(id);
-						} else {
-							enabledTools.delete(id);
-						}
-						applyTools();
-						persistState();
-					},
-					() => {
-						// Close dialog
-						done(undefined);
-					},
-				);
-
-				container.addChild(settingsList);
-
-				const component = {
-					render(width: number) {
-						return container.render(width);
-					},
-					invalidate() {
-						container.invalidate();
-					},
-					handleInput(data: string) {
-						settingsList.handleInput?.(data);
-						tui.requestRender();
-					},
-				};
-
-				return component;
-			});
-		},
-	});
-
-	// Restore state on session start
-	pi.on("session_start", async (_event, ctx) => {
-		restoreFromBranch(ctx);
-	});
-
-	// Restore state when navigating the session tree
-	pi.on("session_tree", async (_event, ctx) => {
-		restoreFromBranch(ctx);
-	});
-
-	// Restore state after branching
-	pi.on("session_branch", async (_event, ctx) => {
-		restoreFromBranch(ctx);
-	});
-}