feat(deepseek): fork a DeepSeek session at the message the user picked

deepseek-acp 0.8.0 reads the same `_meta.jetbrains.air.fork` block
claude-agent-acp and codex-acp do, so "fork from here" now cuts a
DeepSeek session's history at the chosen reply instead of silently
forking at the tail.

It is the one adapter that resolves both halves of the point, so both
are sent. Its id side accepts the session log's own `message.id`, which
the parser now records as the fork point of the turn that message
opened — any message in a turn resolves to that same turn, and naming
the opening one is what the Claude parser already does. Its fingerprint
side hashes the history twice, once per assistant message and once per
whole turn, and refuses when the two land on different turns; codeg
renders one bubble per log turn, so the per-turn reading is the one that
matches, and an id that resolves short-circuits before the ambiguous
case can be reached. A turn with neither an id nor text names nothing
and still falls back to a tail fork, as does every agent that reads no
fork point at all.

The pin moves to 0.8.0. Its dependencies are byte-identical to 0.7.0 and
`initialize` moved only its version string, so the entry's capability
claims and the parser's log layout both hold. `messageId` now rides on
message, thought and user chunks; it is inert here, because the schema
crate keeps that field behind a feature this build does not enable and
drops what it does not recognise.