Implement adapter-backed social post command

[Robertg761]

sh1pt promote social post is documented as the cross-posting command for connected social platforms, but today it only prints a stub message.

Scope:

• build a SocialPost from the CLI options (--body, --title, --hashtags, --media, --link, --schedule)
• resolve requested platforms from --platform, or configured social-* adapters when omitted
• support a safe --dry-run preview that shows adapted post content without publishing
• for live runs, load the relevant social adapter, read saved adapter config, connect, and call post()
• add a focused CLI regression test for the command surface

This is one slice of the broader CLI implementation work requested in #133.