First slice of promoting the shared agent toolkit out of the misnamed
`new_chat` package into the cross-agent `app/agents/shared` kernel.
`errors.py` is a leaf module (no intra-package deps) consumed by the
multi-agent chat, the chat streaming flows/monolith, and tests — i.e. it is
shared infrastructure, not single-agent code. Moved it verbatim to
`app.agents.shared.errors` and flipped all 12 importers. No re-export shim
remains since zero importers needed it.
Behavior-preserving: identical class/enum definitions; only the import path
changes. 1208 agent + chat-task tests green.
0354f73f29