Add release management operations to releases toolset

### Describe the feature or problem you'd like to solve

The current toolset has no way to create, update, or delete GitHub releases. Agents can read repository tags and commits, but can't actually publish releases or attach assets.

I hit this while trying to automate a changelog + release workflow. My agent could draft the release notes by reading commit history, but had no way to actually publish the release on GitHub.

### Proposed solution

Add release lifecycle operations:

| Tool | REST API Endpoint |
|------|-------------------|
| `create_release` | `POST /repos/{owner}/{repo}/releases` |
| `update_release` | `PATCH /repos/{owner}/{repo}/releases/{release_id}` |
| `delete_release` | `DELETE /repos/{owner}/{repo}/releases/{release_id}` |
| `upload_release_asset` | `POST /repos/{owner}/{repo}/releases/{release_id}/assets` |

This mirrors the existing pattern for issues (`create_or_update_issue`) and PRs.

### Example prompts or workflows

1. "Create a new release v2.1.0 with these changelog notes"
2. "Upload the build artifacts to the latest release"
3. "Mark release v2.0.0-beta as a prerelease"
4. "Delete the accidental draft release I just created"
5. "Bump the release from draft to published after CI passes"

### Additional context

The `releases` read operations already exist. Happy to submit a PR if this feature is accepted.