refactor(abilities): expose registration backend seam

[chubes4]

Summary

• add a generic filter over DMC ability registration arguments
• let integrations replace execution callbacks while retaining canonical schemas, permissions, metadata, and CLI routing
• verify one adapter replaces the complete workspace-worktree-* family without changing non-worktree abilities
• fix the integration worker mode so the existing closed-lifecycle scenario exercises its intended path

Closes part of #1243.

Verification

• php tests/workspace-ability-registration-integration.php
• php tests/ability-registry-lifecycle.php
• php -l inc/Abilities/AbilityRegistry.php
• php -l tests/workspace-ability-registration-integration.php
• git diff --check

The repo-native full homeboy review test attempt stopped in the WordPress extension harness with infrastructure exit 143 at its 30-second standalone-test command deadline after many tests passed; run 613f32f1-1c13-4cfe-88c3-f9fa4a5dda89. The focused changed-surface tests above pass.

AI Assistance

OpenAI GPT-5.6 Sol via OpenCode inspected the existing ability/CLI routing, implemented the generic registration seam and integration coverage, diagnosed the test-harness mode bug, and ran verification. Chris Huber directed the architecture and owns the change.