docs(openspec): add harden-opencode-runtime-integration spec.md

Author: loong10k

openspec/changes/harden-opencode-runtime-integration/specs/chat-streaming/spec.md

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+# Delta for Chat Streaming
+
+## MODIFIED Requirements
+
+### Requirement: Chat text is accumulated from canonical delta events
+
+The chat streaming client MUST append text only from events that represent text deltas for the supported OpenCode version.
+
+#### Scenario: Part delta received
+
+- GIVEN an event with type `message.part.delta`
+- AND its field represents text
+- WHEN the event belongs to the active session
+- THEN the delta value MUST be appended exactly once
+
+#### Scenario: Part snapshot received
+
+- GIVEN an event with type `message.part.updated`
+- WHEN it contains a full part snapshot
+- THEN the snapshot MUST NOT be blindly appended as a new text delta
+
+### Requirement: Prompt submission waits for event-stream readiness
+
+The chat streaming client MUST wait for SSE connection readiness before sending `prompt_async`.
+
+#### Scenario: SSE cannot connect
+
+- GIVEN a session has been selected
+- AND the SSE connection fails before becoming ready
+- WHEN a streaming chat is started
+- THEN `prompt_async` MUST NOT be submitted
+- AND the streaming result MUST fail with the transport error
+
+### Requirement: SSE terminal failures propagate to the chat result
+
+Transport failure after connection MUST complete the corresponding streaming chat exceptionally unless the chat has already reached a terminal state.
+
+#### Scenario: Connection drops during generation
+
+- GIVEN streaming generation is active
+- WHEN the SSE transport fails
+- THEN the streaming result MUST fail without waiting only for the overall read timeout
+
+### Requirement: Local cancellation does not implicitly abort shared server work
+
+Cancelling the local stream MUST release local SSE and timeout resources, while server-side session abort remains explicit.
+
+#### Scenario: Local subscriber cancels
+
+- GIVEN a server-side session is generating
+- WHEN one local subscriber cancels its stream
+- THEN the local subscription MUST close
+- AND the SDK MUST NOT implicitly invoke session abort