feat(core): implement linear semantic topic grouping system

Introduces a 'create_new_topic' tool to organize agent work into logical chapters in the UI.

Key changes:
- Added TopicManager singleton to track active session phase.
- Updated Scheduler to prioritize topic markers at the start of batches.
- Injected active topic context into prompt footers for model alignment.
- Removed 'Explain Before Acting' mandate in favor of semantic headers.
- Added behavioral eval to ensure model compliance with grouping.

plans/topic-grouping.md

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+# Plan: Linear Semantic Topic Grouping (Chapters)
+
+Implement a semantic topic grouping system to organize tool calls into
+sequential "Chapters" in the UI using a dedicated tool and prompt feedback.
+
+## 1. Core Component: Topic Management
+
+Create a lightweight `TopicManager` to maintain the session's current semantic
+state.
+
+- **Location:** `packages/core/src/tools/topicTool.ts`
+- **Role:** A singleton that stores the `activeTopicTitle`.
+- **Logic:**
+  - `setTopic(title: string)`: Updates the current title.
+  - `getTopic()`: Returns the current title (defaults to `undefined`).
+
+## 2. The `create_new_topic` Tool
+
+A nearly No-OP tool that acts as the trigger for UI "Chapter" breaks.
+
+- **Name:** `create_new_topic`
+- **Parameters:** `title: string` (e.g., "Researching Codebase", "Implementing
+  Fix").
+- **Execution Logic:**
+  - Calls `TopicManager.setTopic(title)`.
+  - Returns a simple confirmation message: `Topic changed to: "${title}"`.
+- **UI Impact:** The UI detects this tool name in the stream and renders a
+  visual divider/header.
+
+## 3. Scheduler Ordering (Turn-Ahead Alignment)
+
+Ensure the "Chapter Header" appears before actions in a single turn.
+
+- **Location:** `packages/core/src/scheduler/scheduler.ts`
+- **Change:** In `_startBatch`, sort incoming `toolCalls`. Move
+  `create_new_topic` to index `0`.
+- **Reason:** Correct UI rendering order for simultaneous calls.
+
+## 4. Context Reinjection (Loop Feedback)
+
+Keep the model aware of its current "Chapter" to prevent redundant calls.
+
+- **Location:** `packages/core/src/prompts/promptProvider.ts`
+- **Change:** Append the current topic to the system prompt footer (e.g.,
+  `[Active Topic: Researching Auth Flow]`).
+- **Instruction:** Add mandate: _"If the current active topic no longer
+  describes your current phase of work, use `create_new_topic` to start a new
+  chapter."_
+
+## 5. System Prompt Refinement
+
+Update `packages/core/src/prompts/snippets.ts` with guidance.
+
+- **Guidance:** "Use `create_new_topic` to organize your work into logical
+  chapters. Call it when transitioning between major steps (e.g., Research ->
+  Strategy -> Implementation)."
+- **Constraint:** Forward-only semantic marker.
+
+## 6. Verification & Evaluation
+
+- **Behavioral Eval:** Create `evals/topic_grouping.eval.ts`.
+- **Validation:** Run `npm run preflight` to ensure monorepo-wide stability.