Implement — autonomous, worktree-isolated, DAG-parallel implementation

Autonomously implement a `tasks/`-based backlog (as produced by `/ingest`) by fanning out one worktree-isolated sub-agent per task along the dependency DAG, gating each on a green test/build AND a cross-lineage review (GPT-5 finds defects, a Fable adjudicator judges them), with a Sonnet→Opus→Fable→GPT-5 escalation ladder, then merging everything back into the base branch and cleaning up. Use when the user runs `/implement`, or asks to "implement the tasks/backlog", "build out the specs", "work through tasks/ in parallel", or "execute the implementation plan". Local CLI/Desktop only — it creates git worktrees, runs the project's tests, and calls the Codex plugin for review…