Metadata support + connection update + user groups

[dzucconi]

Summary

• Add metadata (entity) and per-connection metadata across block,
  channel, add, connect, and connection commands.
• New connection update <id> --metadata and user groups <slug>
  subcommands; corresponding schema/types added.
• Display metadata in channel, connection, and block views.
• Migrate create-block and create-connection to the preferred
  channels: [{id, position, metadata}] request shape.

Metadata flag

--metadata and --connection-metadata accept either:

• JSON: --metadata '{"status":"reviewed","score":0.95}'
• key=value list: --metadata status=reviewed,score=0.95,featured=true

Parser notes:

• Backslash-escape commas to embed them: note=hello\,world.
• Numbers only coerce when lossless; 00123 and 1.10 stay strings.
• null values allowed for update/merge endpoints to remove keys.