1.34.0: repo-grounded specs, Contracts and migration gates, configurable spec reviewer

• A spec is checked against the repo, not only against itself. Ready specs reject a path:line that does not exist (new files need no line), require Contracts for any external surface even with one task, and require Migration and rollback when data or a stored format outlives the change. A section that does not apply is one line, none — why.
• The pre-T1 spec review knows what to check. It now opens the paths the spec names, checks test commands against the repo, and flags missing Contracts, migrations and threat answers instead of only judging the prose. specReviewer: {"model": "provider/id", "thinking": "high"} runs the same pass on another model, such as the implementer's; unset keeps the session model.
• The implementer validates the plan before T1. The build asks it to check the spec's paths, interfaces, test commands and assumptions against the repo, and stop rather than code around a wrong spec.