feat: Add /history and /delete slash commands for session management

[KillerQueen-Z]

Motivation：
Currently, there is no way for a user to inspect or manage the conversation history within an interactive session. This makes it difficult to:

• Understand the full context being sent to the model.
• Remove irrelevant or erroneous turns to guide the AI's future responses.
• Prune the context to save on tokens for long-running sessions.
  This PR introduces two new slash commands, /history and /delete , to address these issues by providing direct control over the session's dialogue history.
  Implementation
  This change adds two new "direct" slash commands to src/agent/commands.ts :

1. /history

• Displays the complete dialogue history for the current session.
• Each turn is numbered (e.g., [1] , [2] ) for easy reference in the /delete command.
• Clearly labels the role of each turn as [user] or [modelName] .
• Includes a hint informing the user how to use the /delete command.

2. /delete

• Removes one or more turns from the history array based on their number. The modified history is then used for all subsequent model calls.
• Supports flexible formats for specifying turns to delete:
  • Single turn: /delete 3
  • Comma-separated list: /delete 2,5,8
  • Ranges: /delete 4-7
  • A combination of all: /delete 2,4-6,9
• Technical Detail: To prevent index-shifting issues during removal, the command processes indices in descending order, ensuring correctness when deleting multiple turns.
• Context-Awareness: After modifying the history, the command calls resetTokenAnchor() to ensure the agent's token-counting mechanism remains accurate.
  All logic is self-contained within src/agent/commands.ts and does not affect other parts of the application.

How to Test

1. Start an interactive session ( runcode ).
2. Have a multi-turn conversation with the model.
3. Run /history .
   • Expected: The full, numbered conversation history is displayed.
4. Run /delete to remove a specific turn (e.g., /delete 2 ).
   • Expected: A confirmation message is shown.
5. Run /history again.
   • Expected: The specified turn is no longer present in the history.
6. Ask a follow-up question that would have depended on the context of the deleted turn.
   • Expected: The model responds as if the deleted turn never happened, confirming the context was successfully modified.