feat(project): add --label flag to project create and update

[KinomotoMio]

Summary

• Adds repeatable --label <label> to linear project create and linear project update for Linear ProjectLabel IDs.
• Resolves project labels case-insensitively and creates missing labels via projectLabelCreate without forcing a color.
• Keeps project update --label additive by preserving current project label IDs and appending new ones.
• Adds interactive project label selection for project create -i, including a create-new option.

Test plan

• npx --yes deno@2.7.9 task codegen
• npx --yes deno@2.7.9 task check
• npx --yes deno@2.7.9 lint
• TZ=UTC npx --yes deno@2.7.9 task test (337 passed, 5 ignored)
• TZ=UTC npx --yes deno@2.7.9 test --allow-all --quiet test/commands/project/project-create.test.ts test/commands/project/project-update.test.ts

[schpet]

Thanks for this, @KinomotoMio — closing as superseded, but your project update --label work lives on.

Here's how it shook out:

• project create --label — the create-side of this PR shipped independently in feat(project): support overview content on create #216 (which exposed --label/--member/--priority/etc. on project create), so that half is already on main.
• project update --label — this was the genuinely-missing piece, and it just landed in feat(project): add --label to project update #243, reshaped to match the repo's existing label conventions: replace semantics (consistent with project update --team and issue update --label), case-insensitive resolution, and NotFound on a missing label rather than auto-creating. We grounded the replace-vs-additive-vs-delta decision in prior art (gh --add-label/--remove-label, glab --label/--unlabel, kubectl label) — happy to revisit gh-style add/remove deltas as a future repo-wide change across issue + project update.

The projectLabelCreate auto-create and interactive "create new label" behavior from this PR were intentionally dropped to keep labels consistent with the rest of the CLI (nothing else auto-creates labels). Your contribution is credited via Co-authored-by on #243. 🙏