Seperate files for each diffrent social media

[arash77]

Proposal to create separate files for each social media platform (e.g., bluesky_post.md, mastodon_post.md, linkedin_slack_matrix_post.md).
This will allow us to customize content according to each platform's requirements, like Bluesky's lower character limit.

[arash77]

@wm75 @bgruening
Right now it will be separated in galaxy-social-assistent into bluesky and the rest of the platforms.
To make it shorter in Bluesky we can summarize it (use only the title and URL) or create separate PR to handle Bluesky manually later.