feat: add content block presentation metadata

[asherdale]

• add opaque metadata to persisted content blocks and expose it through agent event APIs without changing the model-visible content
• support reserved omnara_hidden and omnara_display_text presentation hints in the web transcript
• omnara_hidden is used to hide Slack routing context, while omnara_display_text is used for preserving canonical mentions and channel references for the model without displaying slack's internal IDs in our web UI
• show message actors in agent conversations and distinguish Slack actors from Omnara users

example:

[cmsparks]

lgtm

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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