feat(coding-agent): add interactive-shell.ts extension example

Demonstrates running interactive commands (vim, git rebase, htop, etc.)
with full terminal access using user_bash event and ctx.ui.custom().

- Auto-detects interactive commands from built-in list
- !i prefix to force interactive mode
- Configurable via INTERACTIVE_COMMANDS/INTERACTIVE_EXCLUDE env vars

closes #532

packages/coding-agent/docs/extensions.md

@@ -543,7 +543,7 @@ pi.on("user_bash", (event, ctx) => {
 });
 ```
 
-**Examples:** [ssh.ts](../examples/extensions/ssh.ts)
+**Examples:** [ssh.ts](../examples/extensions/ssh.ts), [interactive-shell.ts](../examples/extensions/interactive-shell.ts)
 
 ## ExtensionContext