refactor: remove dead leaf modules (v1 domain cleanup, part 1)

Delete confirmed zero-importer dead modules (credential_audit/validator, phase_manager, project_status, github_issue_sync, quality_gate_tool, sdk_hooks, ui/auth, subagent_generator, notifications router/desktop, tasks/ + context/ packages, db backup) + 12 tests. ~11.6k lines. v2 suite green. PR 3 of v1 removal.

Author: frankbria