fix(core): defer plugin load to plugins_loaded hook

[faisalahammad]

Summary

WPCOM_Liveblog::load() was called directly at the bottom of liveblog.php, so it ran the instant PHP parsed the file, before other plugins had a chance to register callbacks. This meant the liveblog_features filter fired too early for a normal add_filter( 'liveblog_features', ... ) call elsewhere to ever take effect, depending on plugin load order.
Fixes #602

Changes

liveblog.php

Before:

	}
	WPCOM_Liveblog::load();

	/** Plupload Helpers */

After:

	}

	// Hooked instead of called directly so that other plugins get a chance to
	// register their own callbacks (e.g. on `liveblog_features`) before we run.
	add_action( 'plugins_loaded', array( 'WPCOM_Liveblog', 'load' ), 0 );

	/** Plupload Helpers */

Why: hooking onto plugins_loaded at priority 0 defers load() until after every active plugin has been included, so any plugin's own top-level add_filter() call is guaranteed to be registered by the time Liveblog applies its filters. Priority 0 keeps it running as early as possible on that hook, same as before relative to init.

tests/Integration/LiveblogLoadTest.php

New test asserting WPCOM_Liveblog::load is registered on plugins_loaded at priority 0. Fails against the old code, where load() was never registered as a callback at all.

Testing

Test 1: liveblog_features filter now applies

1. Added a small test plugin with add_filter( 'liveblog_features', function( $f ) { return array( 'commands', 'emojis', 'authors' ); } ); (no hashtags), activated as a regular plugin.
2. Opened a post with an active liveblog and typed #something in the entry composer.
   Result: hashtag suggestion no longer triggers, other features (commands/emoji/authors) still work. Before the fix, the filter had no effect regardless of plugin activation order.

Test 2: automated suite

• composer test:unit — 7/7 passing
• composer test:integration (via wp-env) — 158/158 passing, including the new LiveblogLoadTest
• composer lint and composer cs clean on changed files