Recursive shared-folder rule: a shared/ must be shared by ALL siblings at its
level. The kernel (context, compaction, retry_after, web_search) was shared by
only 2 of the agents -- anonymous_chat + multi_agent_chat -- never by podcaster
or video_presentation. Those 2 are the "chat" category, so their shared code
belongs in that category's shared/, not the top-level one.
app/agents/anonymous_chat/ -> app/agents/chat/anonymous_chat/
app/agents/multi_agent_chat/ -> app/agents/chat/multi_agent_chat/
app/agents/shared/ -> app/agents/chat/shared/ (anon<->mac kernel)
Top-level app/agents/shared/ is gone: nothing was shared across all three
categories (chat / podcaster / video_presentation).
~289 import sites rewritten (app.agents.{anonymous_chat,multi_agent_chat,shared}
-> app.agents.chat.*); all moves are git renames (history preserved).
app/agents/ now: chat/, podcaster/, video_presentation/, runtime/.
