Add configurable reviewer handoff for PR delivery

[Fuiste]

Summary

• add configurable [handoff] settings for PR-body overlays, provenance detail, optional managed comments, and repo-owned snippet paths
• persist Night Shift handoff state and render deterministic PR handoff/body/comment updates without clobbering non-Night-Shift content
• document the new delivery behavior and extend QA/test coverage for config parsing, handoff rendering, GitHub updates, and persistence

Testing

• gleam test

[Fuiste]

I found two blocking issues in the handoff flow.

• src/night_shift/infra/task_delivery.gleam:67-92 renders the handoff body before github.open_or_update_pr returns the PR identity, but src/night_shift/domain/pr_handoff.gleam:128-133 prints task.pr_number in the default stack-context block. On first delivery task.pr_number is still empty, so new PRs will publish PR: (none) in the handoff body until a later redelivery updates it.
• src/night_shift/github.gleam:107-128 only searches the result of issue_comments, and src/night_shift/github.gleam:384-399 fetches a single /issues/{n}/comments page with no pagination. Once a PR has enough issue comments for the managed handoff comment to fall off page 1, upsert_handoff_comment will stop finding it and create duplicates, which breaks the "update in place instead of adding comment noise" contract documented in this PR.

[Fuiste]

QA completed on the combined stack branch and the work has been superseded by #35.

What I validated:

• combined-branch end-to-end QA in the scratch repo
• intentional interruption followed by doctor, resume --explain, and resume
• successful verification, provenance/runtime artifacts, and delivery on the resumed run

Closing this PR in favor of the QAed combined branch.