* fix(pi): stream assistant deltas when pi omits the cumulative message
pi 0.84.0 removed the cumulative `message` snapshot from JSON/RPC
`message_update` events, so `handleMessageUpdate` dereferenced
`undefined` and took down the whole daemon on the first assistant delta.
Every prompt to a pi agent crash-looped the worker.
`message_update` carries an `assistantMessageEvent`, which pi only emits
for assistant messages, so a missing snapshot is treated as an assistant
update. `message_start` still carries the message, so the response id
keeps flowing through `activeAssistantMessageId`.
Fixes#2940Fixes#2946Fixes#2950
* fix(pi): give the message_update COMPAT tag a version and removal date
Follows docs/protocol-compatibility.md: every shim tag needs a name, a
version, and a removal condition or date so `rg "COMPAT\("` stays a
schedulable cleanup backlog.
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Co-authored-by: dundunHa <dundunhade@gmail.com>
