fix: flush bracket rewrite rules on activation

[TJCurnutte]

Summary

• Registers the brackets custom post type during plugin activation before flushing rewrite rules.
• Flushes rewrite rules once on deactivation so stale bracket routes are removed.
• Keeps rewrite flushing out of normal request handling.

Why

flush_rewrite_rules() was already called in activate(), but the plugin's normal bracketpress_init hook does not run during activation. That can flush rules before register_post_type( 'brackets', ... ) has populated the bracket permalink structure, leaving bracket links 404ing until a site owner manually saves permalinks.

Verification

• git diff --check
• Static activation-flow check confirmed $this->register_posts(); runs before flush_rewrite_rules();
• Static deactivation-flow check confirmed deactivation flushes rewrites once

Note: php -l was not available in my local Mac environment.

Closes #7