feat(issue): add attachment read downloads (#218)

- Add a read-only `attachment_read` tool for issue and issue comment attachments.
- Use the existing Gitea SDK attachment metadata APIs for `list` and `get`.
- Add authenticated `/attachments/{uuid}` downloads in the MCP server for `download`, with small image inline results and streamed file downloads for larger content.
- Add `-max-inline-attachment-bytes` / `GITEA_MAX_INLINE_ATTACHMENT_BYTES` to configure inline image limits.
- Update the tool tables in all README variants.

Refs: #209

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/218
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: yp05327 <576951401@qq.com>

README.md

@@ -151,6 +151,7 @@ Once configured, try `list all my repositories` in the chat box.
 | package_read                 | Packages     | Read   | Read package registry: list packages, list versions, or get a version                      |
 | package_write                | Packages     | Write  | Delete a package version (irreversible)                                                    |
 | list_issues                  | Issue        | Read   | List repository issues                                                                     |
+| attachment_read              | Issue        | Read   | Read issue/comment attachments: list metadata, get metadata, or download content           |
 | issue_read                   | Issue        | Read   | Read issue: details, comments, or labels                                                   |
 | issue_write                  | Issue        | Write  | Write issues: create, update, manage comments and labels                                   |
 | list_pull_requests           | Pull Request | Read   | List repository pull requests                                                              |