Introduce a Quality Gate stage and related plumbing plus a retry/backoff for PR creation. Added quality_gate spec and implementation (QualityFeedback, auto-fix + check phases) and integrated it into the validation pipeline (PipelineOutcome.FAIL_QUALITY, PipelineResult.quality_feedback, worktree support). Updated prompt.build_prompt to include Previous Quality Gate Feedback and engine nodes to route FAIL_QUALITY like FAIL_V3 and surface quality diagnostics in halt reports. Also added exponential-backoff retries for `gh pr create` with transient error classification in scm/pr.py. Tests, planning artifacts, and sprint status updated accordingly.