cli-23: Remove auto-compaction on clear/exit and other improvements

[szmania]

Summary

This PR implements the removal of auto-compaction when clearing or exiting the session, along with several other architectural refinements and reverts to synchronous operations for stability.

Key Changes

• Auto-Compaction Optimization: Context compaction is now skipped when the user issues /clear, /exit, or /quit commands, preventing wasteful computation.
• Synchronous Linter & Commands: Reverted the Linter and shell command execution from async to synchronous (using asyncio.to_thread for run_cmd). This simplifies the codebase and improves reliability in interrupt handling.
• Nested Delegation Removal: Removed the allow_nested_delegation feature. Sub-agents are no longer permitted to delegate to further sub-agents.
• MCP Lifecycle Management: Enhanced the logic for disconnecting the "Local" MCP server when switching between Agent and non-Agent coders.
• Notification Refinement: Removed bell notifications and notification cooldown logic from the IO layer.
• Testing: Added comprehensive tests for the new compaction skipping logic and updated existing tests to reflect the synchronous linter changes.

Why Needed

• Auto-compaction before a clear/exit was redundant and consumed unnecessary tokens/time.
• Reverting to synchronous operations for certain tasks reduces complexity and potential race conditions in the async event loop.
• Simplifying sub-agent capabilities ensures a more predictable agent hierarchy.