feat: add appkit codemod on-plugins-ready CLI

[MarioCadenas]

Summary

Adds appkit codemod on-plugins-ready CLI command to auto-migrate apps from the old autoStart/extend/start pattern to the new onPluginsReady callback.

Depends on #280.

Usage

npx appkit codemod on-plugins-ready              # dry-run, auto-detect
npx appkit codemod on-plugins-ready --write      # apply changes
npx appkit codemod on-plugins-ready --path file  # specific file

What it does

• Pattern A (.then chain): Extracts callback body, removes .start(), injects as onPluginsReady property, preserves .catch() handlers (including arrow functions with parens)
• Pattern B (await + imperative): Gathers extend/start calls, injects into onPluginsReady, removes standalone statements. Bails with warning for complex cases (non-server usage of appkit handle, multiple extend calls)
• autoStart stripping: Removes autoStart: true/false from all server() configs, preserves other properties
• File discovery: Auto-detects files importing createApp from @databricks/appkit
• Idempotent: No-ops with friendly message if already migrated
• Dry-run by default: --write flag to apply changes

Test plan

• 9 tests covering both patterns, bail-outs, idempotency, autoStart stripping, arrow function catch handlers, multi-extend bail
• Full suite passes (1595/1595)
• Typecheck passes
• Build passes

Demo

codemod-demo.mp4