feat: add opencode provider and refactor stream model to head/tail architecture

.tasks/9b83b90a.md

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+---
+id: 9b83b90a
+title: Refactor stream buffering to head/tail model
+status: open
+deps: []
+created: 2026-01-11T08:48:35.444Z
+---
+
+## Goal
+
+Replace the current special-case buffering logic with a unified head/tail model where:
+- **Tail**: Committed StreamItem[] (history)
+- **Head**: Active StreamItem[] (being streamed, same type as tail)
+
+Both use the same `reduceStreamUpdate()` function. The only difference is that head items get flushed to tail when a stream item completes.
+
+## Current Problems
+
+1. `applyStreamEventWithBuffer()` has hardcoded assistant_message buffering
+2. `StreamingBufferEntry` is a different type than `StreamItem`
+3. Reasoning chunks go through reducer directly (no buffering)
+4. Duplicate logic between `appendAssistantMessage()` and buffer handling
+
+## Design
+
+### Types
+
+```typescript
+// Head is just StreamItem[] - no special type needed
+// Tail is just StreamItem[] - same as before
+
+interface StreamState {
+  tail: StreamItem[];  // committed history
+  head: StreamItem[];  // active streaming items
+}
+```
+
+### Core Function
+
+```typescript
+function applyStreamEvent(params: {
+  tail: StreamItem[];
+  head: StreamItem[];
+  event: AgentStreamEventPayload;
+  timestamp: Date;
+}): {
+  tail: StreamItem[];
+  head: StreamItem[];
+  changedTail: boolean;
+  changedHead: boolean;
+}
+```
+
+### Flush Logic
+
+Head items are flushed to tail when:
+1. A different item kind arrives (thought → tool_call flushes the thought)
+2. Turn completion events (turn_completed, turn_failed, turn_canceled)
+3. User message arrives (new turn starting)
+
+### Implementation Steps
+
+1. Remove `StreamingBufferEntry` type
+2. Remove `StreamingHead` type (or repurpose)
+3. Modify `applyStreamEventWithBuffer` → `applyStreamEvent` to work with head/tail
+4. Head uses same `reduceStreamUpdate` as tail
+5. Add flush logic based on item kind transitions
+6. Update session-context.tsx to use new API
+7. Update agent-stream-view.tsx to render head items
+8. Update session-store.ts types
+
+## Flush Conditions
+
+```typescript
+function shouldFlushHead(head: StreamItem[], incomingKind: StreamItemKind): boolean {
+  if (head.length === 0) return false;
+  const lastHeadKind = head[head.length - 1].kind;
+  
+  // Different kind arriving = flush
+  if (lastHeadKind !== incomingKind) return true;
+  
+  return false;
+}
+```
+
+## Rendering
+
+In agent-stream-view.tsx:
+- Tail is rendered in FlatList (reversed, virtualized)
+- Head is rendered in ListHeaderComponent (always visible, not virtualized since it's small)
+
+This is similar to current approach but head can now contain any StreamItem types.