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/dynamic-border.ts

@@ -4,9 +4,9 @@ import { theme } from "../theme/theme.js";
 /**
  * Dynamic border component that adjusts to viewport width.
  *
- * Note: When used from hooks loaded via jiti, the global `theme` may be undefined
+ * Note: When used from extensions loaded via jiti, the global `theme` may be undefined
  * because jiti creates a separate module cache. Always pass an explicit color
- * function when using DynamicBorder in components exported for hook use.
+ * function when using DynamicBorder in components exported for extension use.
  */
 export class DynamicBorder implements Component {
 	private color: (str: string) => string;