Bot Chats created before the epoch mechanism persisted prompts with no
protocol section and no stamp — the staleness check only fires on
stamped prompts, so pre-existing bots would never learn to message
teammates. stored_bot_chat_prompt_needs_upgrade() migrates them: one
rebuild, title-gated to Bot Chat, only when the probe would actually
emit a section (SOUL-append legacies and unmanaged installs are left
alone — rebuilding those would loop). The rebuilt prompt carries the
stamp, so the upgrade can never re-fire.
E2E v3b through the real restore path: legacy Bot Chat upgraded once
then verbatim-reused; legacy regular sessions byte-untouched.
tests/agent/ 4648/4648.
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