fix: defer code ability registration until API loads

[chubes4]

Summary

• Defer DMC ability registration to wp_abilities_api_init when ability classes are constructed before WP_Ability is loaded.
• Preserve immediate registration when the Abilities API is already available.
• Add smoke coverage for the deferred constructor path that wp-codebox exercises.

Verification

• php tests/smoke-late-ability-registration.php
• php tests/smoke-deferred-ability-registration.php
• php -l inc/Abilities/WorkspaceAbilities.php && php -l inc/Abilities/GitHubAbilities.php && php -l inc/Abilities/GitSyncAbilities.php && php -l inc/Abilities/WordPressRuntimeAbilities.php && php -l inc/Abilities/CodeTaskAbilities.php && php -l tests/smoke-deferred-ability-registration.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Diagnosed the wp-codebox canary registration ordering failure, drafted the code change and smoke test, and ran local verification. Chris remains responsible for review and merge.