Clarify that pull_request_review_write create without event creates a pending review

## Problem

When using the `pull_request_review_write` tool from a client like Claude Code to "create a review," the server correctly creates a pending review if the `event` parameter is omitted. However, the user-facing title/description does not clearly indicate that this call **only creates a pending review and does not submit it**.

### Current behavior

- **Tool name:** `pull_request_review_write`
- **Title:** `"Write operations (create, submit, delete) on pull request reviews"`
- **Description (in code)** already says:
> `create: Create a new review of a pull request. If "event" parameter is provided, the review is submitted. If "event" is omitted, a pending review is created.`

Clients often show only the short title or a condensed description, so users just see that a review is being "created," not that it's **pending only**.

This is confusing when watching MCP tool calls in the UI: the first call to `pull_request_review_write` looks like it "created a review," but in GitHub it is actually just opening a **pending** review, which then gets line comments via `add_comment_to_pending_review`, and later is submitted with `pull_request_review_write method: "submit_pending"`.

The underlying workflow is correct; **the wording is misleading**.

### Requested change (minimal, backwards compatible)

Update the **Title annotation** for `pull_request_review_write` to make "pending" explicit, e.g.:

- `"Create pending / submit / delete pull request reviews"`, or
- `"Pending review create / submit / delete for pull requests"`

Optionally, emphasize in the short description that `method: "create"` without `event` creates a **pending** review and does **not** submit it.

This keeps the existing single-tool design (no new tools, no API change) but makes the behavior much clearer to users of MCP clients that only surface the short title/description.