Add --project to document update

[schpetbot]

What

document create accepts --project, but document update didn't — so a document's project attachment was fixed at creation time; changing it meant using the web UI. This adds --project <uuid|slug|name> to document update, reusing the same resolveProjectId path as create (consistent resolution and not-found errors).

linear document update <id> --project F-FOO          # by slug ID
linear document update <id> --project "Some Project" # by name
linear document update <id> --project <uuid>         # by UUID

On the reporter's --remove / multi-link framing

The issue framed this around Linear's web UI supporting multiple project links per document with add/remove semantics (--project X --remove). The API models it differently: a Document has a single related project (the scalar DocumentUpdateInput.projectId; there's no document-links mutation), and document create requires exactly one anchor — so a document always has one project and setting simply replaces it.

I prototyped a --clear-project detach flag, but live testing showed the API silently ignores projectId: null (returns success while keeping the project). Shipping it would have been a silent no-op, so it was dropped rather than lie about detaching. Re-pointing to a different project works and is verified end-to-end.

Tests

Command snapshots for set-by-UUID, set-by-name (resolution + mutation), combined --title + --project, and project-not-found. Manually QA'd against a live workspace (create → change by UUID → change by name → confirmed the API ignores clear).

Fixes #225