fix: support URL parameter in tab new command (#64)

* fix: support URL parameter in tab new command

The CLI was correctly sending the URL parameter when running
`agent-browser tab new <url>`, but the TypeScript daemon was
ignoring it because:

1. The schema didn't include the url field (stripped during validation)
2. The TabNewCommand type didn't have a url property
3. The handler didn't pass the URL to browser.newTab()
4. browser.newTab() didn't accept or use a URL parameter

This fix adds URL support throughout the chain so that
`agent-browser tab new https://example.com` now correctly
opens a new tab and navigates to the specified URL.

Fixes #62

* fix: omit url field when not provided in tab new command

Previously, the CLI always sent "url": null when no URL was provided,
which caused Zod validation to fail with "Expected string, received null".

Now the url field is only included when a URL is actually provided.

Fixes issue reported by @ctate in PR review.

* refactor: move navigation logic from BrowserManager to handleTabNew

Address review feedback:
- Add .min(1) to URL validation for consistency with navigateSchema
- Keep BrowserManager.newTab() simple (single responsibility)
- Handle navigation in handleTabNew following same pattern as handleNavigate

src/protocol.ts

@@ -730,6 +730,7 @@ const closeSchema = baseCommandSchema.extend({
 // Tab/Window schemas
 const tabNewSchema = baseCommandSchema.extend({
   action: z.literal('tab_new'),
+  url: z.string().min(1).optional(),
 });
 
 const tabListSchema = baseCommandSchema.extend({