Integrate OGP generation into generate (--with-ogimage)

[rokuosan]

Scope

Integrate OGP image generation into the generate command.

• Add a --with-ogimage flag to generate
• When set, generate the OGP image at the end of the article generation flow (fetch issue → download/place images → convert)
• Opt-in: when the flag is not set, behavior is unchanged

Out of scope

• The OGP rendering pkg itself (depends on it)
• The ogimage subcommand (depends on it, for reference/consistency)