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>

pkg/flag/flag.go

@@ -7,6 +7,8 @@ var (
 	Version string
 	Mode    string
 
+	MaxInlineAttachmentBytes int
+
 	Insecure     bool
 	ReadOnly     bool
 	Debug        bool