feat: conversation history threading in ModelCompletionRequest

[broomva]

Adds ConversationTurn type and build_conversation_history() to KernelRuntime. Fixes multi-turn memory loss.

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Summary by CodeRabbit

• New Features
  • Model completion requests now carry conversation history, enabling the system to maintain and leverage context from prior user and assistant turns within sessions when communicating with model providers.

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

The pull request extends the protocol to include conversation history tracking. A new ConversationTurn type is introduced and re-exported from the crate root. The ModelCompletionRequest now carries a conversation_history field populated by a new runtime method that reconstructs prior user and assistant turns from persisted events.

Changes

Cohort / File(s)	Summary
Protocol Types crates/aios-protocol/src/lib.rs, crates/aios-protocol/src/ports.rs	Added ConversationTurn struct with role and content fields. Extended ModelCompletionRequest with conversation_history: Vec<ConversationTurn> field (serialized with defaults, omitted when empty). Re-exported ConversationTurn from crate root.
Runtime Implementation crates/aios-runtime/src/lib.rs	Added build_conversation_history() async helper that reads up to 10,000 events from the store, extracts user turns from deliberation summaries and assistant text from messages/deltas, aggregates them into conversation turns (capped at 50 most recent), and passes the result into ModelCompletionRequest during tick_on_branch processing.

Sequence Diagram(s)

sequenceDiagram
    participant Runtime as KernelRuntime
    participant EventStore as Event Store
    participant Provider as Model Provider
    
    Runtime->>Runtime: tick_on_branch(session_id, branch_id)
    Runtime->>EventStore: read_events(session_id, branch_id, seq: 0, limit: 10000)
    EventStore-->>Runtime: events[]
    Runtime->>Runtime: build_conversation_history()
    Note over Runtime: Extract user turns from<br/>DeliberationProposed<br/>Extract assistant text from<br/>Message & TextDelta<br/>Aggregate into turns<br/>Cap at 50 most recent
    Runtime->>Runtime: Create ModelCompletionRequest<br/>with conversation_history
    Runtime->>Provider: complete(request)
    Provider-->>Runtime: response

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Hops with delight at conversation history unbounded,
Events from the past now faithfully rounded,
User and assistant turns dance in a Vec,
Fifty turns deep—what a glorious tech!
The model now remembers our every word's sound, 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding conversation history threading to ModelCompletionRequest. It is specific, concise, and reflects the primary objective of the PR.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/conversation-history

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