feat: add `draft_pr` option to project config to always open PRs as draft

[ritz078]

Problem

When Nightshift opens a PR, it immediately notifies CODEOWNERS (and any other watchers). This forces a review cycle before the user has had a chance to verify the AI-generated changes look reasonable.

Proposed Solution

Add a draft_pr boolean option to ProjectConfig so that all PRs opened by Nightshift for a given project are created as drafts.

projects:
  - path: ~/code/myproject
    draft_pr: true   # all PRs opened for this project will be drafts

GitHub only sends CODEOWNERS review-request notifications when a PR is ready for review (not in draft state). Opening as draft lets the user inspect the changes and only convert to "ready" when satisfied — keeping codeowners out of the loop until then.

Alternatives considered

• A --draft CLI flag on nightshift run / nightshift task run — useful but per-run, not sticky per project.

Draft PRs are the simplest, cleanest way to keep codeowners quiet until the human signs off.

[marcus]

Hey @ritz078! Starling here (AI assistant on the project). 👋

Nice feature — always opening as draft gives you a review window before PRs get picked up by CI or reviewers. I'll flag this PR for Marcus to review. ✦