feat(project): add --label to project update

[schpet]

Adds a repeatable --label flag to linear project update, mirroring the existing project create --label.

Behavior

• Repeatable --label <label> ({ collect: true }, no short alias — -l is --lead on project commands).
• Replace semantics: the supplied labels become the project's complete label set, consistent with project update --team and issue update --label.
• Names resolve case-insensitively; an unknown label raises NotFoundError (no auto-create).
• Empty/whitespace labels are rejected up front; case-insensitive duplicates collapse to a single ID.
• Label groups are excluded from the lookup (isGroup: { eq: false }) so a group name can't be sent as a non-assignable label id — this also hardens the existing project create --label.

Design note

The --label semantics (replace vs additive vs gh-style --add/--remove deltas) were chosen by grounding in prior art (gh issue/pr edit use --add-label/--remove-label; glab issue update uses --label/--unlabel; kubectl label is delta-oriented) against this repo's own conventions. Replace won because the closest siblings in-repo — project update --team (same command) and issue update --label — both replace; adopting gh-style deltas on one command alone would be inconsistent (a possible future repo-wide enhancement). Additive-only can never remove a label, so it was rejected.

The project-label lookup is extracted from project-create into a shared getProjectLabelIdByName helper in utils/linear.ts so create and update stay identical.

Relationship to #226

This reshapes the project update half of #226 to the repo's existing label conventions. The project create --label half of #226 already shipped in #216, and #226's auto-create / interactive-create behavior is intentionally dropped in favor of NotFound-on-missing. #226 will be closed as superseded; credit retained via Co-authored-by.

Tests

project update: replace (asserts exact labelIds sent), case-insensitive dedup, NotFound-before-mutation, empty-label validation, --label-alone, and no-option. Regenerated references/project.md and the update snapshot. Full suite green under Deno 2.7.9 + TZ=UTC (keyring integration excluded, as in CI).