Story 15: Implement New Session cancellation

- Call cancel_chat before clearing session state
- Prevents tools from executing silently in background
- Prevents streaming from leaking into new session
- Uses same cancellation infrastructure as Story 13
- Clean session start with no side effects

Closes Story 15

.living_spec/stories/archive/15_new_session_cancellation.md

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+# Story 14: New Session Cancellation
+
+## User Story
+**As a** User
+**I want** the backend to stop processing when I start a new session
+**So that** tools don't silently execute in the background and streaming doesn't leak into my new session
+
+## The Problem
+
+**Current Behavior (THE BUG):**
+1. User sends message → Backend starts streaming → About to execute a tool (e.g., `write_file`)
+2. User clicks "New Session" and confirms
+3. Frontend clears messages and UI state
+4. **Backend keeps running** → Tool executes → File gets written → Streaming continues
+5. **Streaming tokens appear in the new session**
+6. User has no idea these side effects occurred in the background
+
+**Why This Is Critical:**
+- Tool calls have real side effects (file writes, shell commands, searches)
+- These happen silently after user thinks they've started fresh
+- Streaming from old session leaks into new session
+- Can cause confusion, data corruption, or unexpected system state
+- User expects "New Session" to mean a clean slate
+
+## Acceptance Criteria
+
+- [ ] Clicking "New Session" and confirming cancels any in-flight backend request
+- [ ] Tool calls that haven't started yet are NOT executed
+- [ ] Streaming from old request does NOT appear in new session
+- [ ] Backend stops processing immediately when cancellation is triggered
+- [ ] New session starts with completely clean state
+- [ ] No silent side effects in background after new session starts
+
+## Out of Scope
+- Stop button during generation (that's Story 13)
+- Improving the confirmation dialog (already done in Story 20)
+- Rolling back already-executed tools (partial work stays)
+
+## Implementation Approach
+
+### Backend
+- Uses same `cancel_chat` command as Story 13
+- Same cancellation mechanism (tokio::select!, watch channel)
+
+### Frontend
+- Call `invoke("cancel_chat")` BEFORE clearing UI state in `clearSession()`
+- Wait for cancellation to complete before clearing messages
+- Ensure old streaming events don't arrive after clear
+
+## Testing Strategy
+
+1. **Test Tool Call Prevention:**
+   - Send message that will use tools (e.g., "search all TypeScript files")
+   - Click "New Session" while it's thinking
+   - Confirm in dialog
+   - Verify tool does NOT execute (check logs/filesystem)
+   - Verify new session is clean
+
+2. **Test Streaming Leak Prevention:**
+   - Send message requesting long response
+   - While streaming, click "New Session" and confirm
+   - Verify old streaming stops immediately
+   - Verify NO tokens from old request appear in new session
+   - Type new message and verify only new response appears
+
+3. **Test File Write Prevention:**
+   - Ask to write a file: "Create test.txt with current timestamp"
+   - Click "New Session" before tool executes
+   - Check filesystem: test.txt should NOT exist
+   - Verify no background file creation happens
+
+## Success Criteria
+
+**Before (BROKEN):**
+```
+User: "Search files and write results.txt"
+Backend: Starts streaming...
+User: *clicks New Session, confirms*
+Frontend: Clears UI ✓
+Backend: Still running... executes search... writes file... ✗
+Result: File written silently in background ✗
+Old streaming tokens appear in new session ✗
+```
+
+**After (FIXED):**
+```
+User: "Search files and write results.txt"
+Backend: Starts streaming...
+User: *clicks New Session, confirms*
+Frontend: Calls cancel_chat, waits, then clears UI ✓
+Backend: Receives cancellation, stops immediately ✓
+Backend: Tools NOT executed ✓
+Result: Clean new session, no background activity ✓
+```
+
+## Related Stories
+- Story 13: Stop Button (shares same backend cancellation mechanism)
+- Story 20: New Session confirmation dialog (UX for triggering this)
+- Story 18: Streaming Responses (must not leak between sessions)