Feature request: import/create comment threads on Artifacts (round-trip serialization)

## Feature request

Allow programmatically creating/importing comment threads on a published Artifact — the write-side counterpart of the existing `comments` read action on the Artifact tool.

## Current behavior

- Comment threads on Artifacts can be **read** (`action: "comments"`) and **replied to** (`action: "reply"`, on activated threads), so Claude can export a durable transcript of threads to disk.
- There is **no way to import** them: threads live server-side, attached to a specific artifact URL, and can only be created by viewers in the UI.

## Why it matters

We use commentable Artifact pages as review surfaces for a multi-agent build workflow (each work track publishes a review page; the human leaves pinned comment threads; Claude acts on them). We serialize each page to disk in-repo: source file + artifact-URL binding + a one-way comment export. That covers durability, but the threads themselves aren't portable:

- If a page is ever recreated at a new URL (lost binding, new account/workspace, migrating a project), all live threads are orphaned — the export is a flat transcript, not restorable state.
- Backup/restore of a project's review history can't round-trip.

## Requested capability

Either of these would solve it:

1. An `import_comments` / `create_thread` action on the Artifact tool (thread anchor selector + comment list with authorship/timestamps, clearly marked as imported), or
2. An export format (JSON) whose re-import onto a same-content artifact restores threads with their anchors and resolved/open state.

## Workaround today

One-way markdown export of threads committed to the repo, and treating the artifact-URL binding file as precious so republishing always targets the same URL.

Start with the Artifact tool's existing `comments` read action and `reply` action, then trace how the artifact URL binding and markdown thread export are represented. Define how imported threads, anchors, authorship, timestamps, and open/resolved state should round-trip, and confirm completion with a JSON export that can restore threads on a same-content artifact.

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