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/src/modes/interactive/components/hook-selector.ts

@@ -1,92 +0,0 @@
-/**
- * Generic selector component for hooks.
- * Displays a list of string options with keyboard navigation.
- */
-
-import { Container, getEditorKeybindings, Spacer, Text } from "@mariozechner/pi-tui";
-import { theme } from "../theme/theme.js";
-import { DynamicBorder } from "./dynamic-border.js";
-
-export class HookSelectorComponent extends Container {
-	private options: string[];
-	private selectedIndex = 0;
-	private listContainer: Container;
-	private onSelectCallback: (option: string) => void;
-	private onCancelCallback: () => void;
-
-	constructor(title: string, options: string[], onSelect: (option: string) => void, onCancel: () => void) {
-		super();
-
-		this.options = options;
-		this.onSelectCallback = onSelect;
-		this.onCancelCallback = onCancel;
-
-		// Add top border
-		this.addChild(new DynamicBorder());
-		this.addChild(new Spacer(1));
-
-		// Add title
-		this.addChild(new Text(theme.fg("accent", title), 1, 0));
-		this.addChild(new Spacer(1));
-
-		// Create list container
-		this.listContainer = new Container();
-		this.addChild(this.listContainer);
-
-		this.addChild(new Spacer(1));
-
-		// Add hint
-		this.addChild(new Text(theme.fg("dim", "↑↓ navigate  enter select  esc cancel"), 1, 0));
-
-		this.addChild(new Spacer(1));
-
-		// Add bottom border
-		this.addChild(new DynamicBorder());
-
-		// Initial render
-		this.updateList();
-	}
-
-	private updateList(): void {
-		this.listContainer.clear();
-
-		for (let i = 0; i < this.options.length; i++) {
-			const option = this.options[i];
-			const isSelected = i === this.selectedIndex;
-
-			let text = "";
-			if (isSelected) {
-				text = theme.fg("accent", "→ ") + theme.fg("accent", option);
-			} else {
-				text = `  ${theme.fg("text", option)}`;
-			}
-
-			this.listContainer.addChild(new Text(text, 1, 0));
-		}
-	}
-
-	handleInput(keyData: string): void {
-		const kb = getEditorKeybindings();
-		// Up arrow or k
-		if (kb.matches(keyData, "selectUp") || keyData === "k") {
-			this.selectedIndex = Math.max(0, this.selectedIndex - 1);
-			this.updateList();
-		}
-		// Down arrow or j
-		else if (kb.matches(keyData, "selectDown") || keyData === "j") {
-			this.selectedIndex = Math.min(this.options.length - 1, this.selectedIndex + 1);
-			this.updateList();
-		}
-		// Enter
-		else if (kb.matches(keyData, "selectConfirm") || keyData === "\n") {
-			const selected = this.options[this.selectedIndex];
-			if (selected) {
-				this.onSelectCallback(selected);
-			}
-		}
-		// Escape or Ctrl+C
-		else if (kb.matches(keyData, "selectCancel")) {
-			this.onCancelCallback();
-		}
-	}
-}