Feature request: batch-fetch merged PR metadata for release notes / changelog generation

### Problem

For a **release window**, teams often have a **known list of merged PR numbers**. Building a **structured changelog** today requires opening each PR or scripting against REST/GraphQL outside MCP. Agents would benefit from **one MCP call** that returns **normalized metadata** for many PRs.

### Proposed solution (v1)

Add a tool that accepts:

- `owner`, `repo`
- `pull_numbers`: array of integers (**bounded**)

**Returns** (exact fields up to maintainers), per PR, for example:

- `number`, `title`, `html_url`
- `merged_at`, `merge_commit_sha` (when available)
- `labels` (at least `name`)
- `author` login

**Output:** JSON suitable for templating Markdown release notes in the client (prose stays in the agent or human editor).

**Explicit non-goals (v1):**

- Full combined diff across PRs (too heavy); linking to each PR is enough.
- Guessing “what shipped in this release” without an explicit PR list—the caller provides the list.

### Why this matters

- **Release engineering + agents:** “Here are the PRs we shipped” → one call → table/sections for notes, Slack, or tickets.
- Composes with `search_pull_requests` / `list_pull_requests`: caller selects PRs, tool **hydrates** details.

### Relationship to existing requests

- **Create/update releases** (#1909) addresses **publishing** a release after notes exist; this request is **read-side batch hydration** of PR metadata to **produce** those notes.

### Related issues

- Issue/PR relationship graph (planning context, different shape): https://github.com/github/github-mcp-server/issues/1510
- Companion (batch review threads): https://github.com/github/github-mcp-server/issues/2301

### Acceptance criteria (suggestion)

- [ ] Bounded input; documented max PR count.
- [ ] Clear behavior for non-merged or missing PRs (per-PR errors vs fail-all—documented).
- [ ] Tests + docs per repo conventions.

### Note

We are happy to prototype a PR if maintainers agree on v1 scope.