Add the channel messages and the usage record to proto

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/proto/tests/fixtures/wire/error_session_full.json

@@ -0,0 +1 @@
+{"v":1,"id":3,"final":true,"msg":{"kind":"error","body":{"code":"session_full","detail":"this conversation is full; start a new one"}}}

crates/proto/tests/fixtures/wire/turn.json

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+{"v":1,"id":3,"final":true,"msg":{"kind":"turn","body":{"session":"chat-1789700000-42","content":"What time is it?","resume":false}}}

crates/proto/tests/fixtures/wire/turn_done.json

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+{"v":1,"id":3,"final":true,"msg":{"kind":"turn_done","body":{"content":"It is noon.","usage":{"cache_n":539,"prompt_n":27,"predicted_n":24,"reasoning_tokens":8,"thinking_capped":false}}}}

crates/proto/tests/fixtures/wire/turn_event_content.json

@@ -0,0 +1 @@
+{"v":1,"id":3,"final":false,"msg":{"kind":"turn_event","body":{"event":"content","text":"It is "}}}

crates/proto/tests/fixtures/wire/turn_event_retrying.json

@@ -0,0 +1 @@
+{"v":1,"id":3,"final":false,"msg":{"kind":"turn_event","body":{"event":"retrying","attempt":2,"after_ms":1500,"error":"the server went silent"}}}

crates/proto/tests/fixtures/wire/turn_event_tool_result.json

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+{"v":1,"id":3,"final":false,"msg":{"kind":"turn_event","body":{"event":"tool_result","name":"clock","class":"public","truncated":false}}}