Saggar integration

Saggar records an agent turn ledger under ~/.saggar/turns/v1/. Fettle reads that ledger to bind a prompt and the resulting file changes into one review target. Accepted or rejected turn reviews can be written to Saggar's local review outbox for the agent to consume.

Without Saggar, repository, branch, and GitHub pull request review still work. Fettle doesn't inherit Saggar credentials or run an agent itself.