feat: Add ability to clear conversation history within the same session

[konokenj]

Feature Request

Add the ability to clear/reset conversation history within the same EC2 instance session without starting a new session.

Use Case

When working on a long session, the conversation history can grow large and may include outdated context that is no longer relevant. Users may want to:

1. Clear history to start fresh with a new task on the same instance
2. Reset context when switching to a different topic
3. Reduce token usage by removing irrelevant historical messages

Current Behavior

• Conversation history is stored in DynamoDB and persists for the entire session
• No way to clear history without starting a new session (new EC2 instance)
• Existing Slack commands: approve_user, dump_history, take_over - none for clearing

Proposed Solution

Option A: Slack Command

Add a clear_history command that clears all messages for the current thread/session.

@remote-swe clear_history

Option B: Web UI Button

Add a "Clear History" button in the session page UI.

Option C: Both

Implement both Slack command and Web UI button for flexibility.

Implementation Considerations

• Should preserve session metadata (repo info, etc.) while clearing messages
• May need confirmation prompt to prevent accidental clearing
• Could optionally keep the last N messages or system context
• Need to handle the case where agent is currently processing

Related Files

• packages/slack-bolt-app/src/app.ts - Slack command handlers
• packages/agent-core/src/lib/messages.ts - Message storage (needs delete function)
• packages/webapp/src/app/sessions/[workerId]/actions.ts - Web UI actions