CLI-29: Robust Interruption in Agent Mode + Various Fixes

[szmania]

Summary

This PR addresses CLI-29: making interruption in agent mode more robust, along with several related fixes and improvements discovered during investigation.

Changes

Core Interruption Fixes

• cecli/helpers/coroutines.py: Removed try/finally wrapper from interruptible_async_generator to prevent CancelledError from being swallowed during cleanup. Removed None check for interrupt_event (always required).
• cecli/coders/base_coder.py: Changed interrupted exception from asyncio.CancelledError back to KeyboardInterrupt for proper propagation. Removed stale interrupt_event.clear() before formatting to avoid re-catching previous interrupts.
• cecli/tui/worker.py: Added KeyboardInterrupt handler to CoderWorker to prevent thread crashes when interruption occurs during TUI operation.

Coder Switching & MCP Lifecycle Fixes

• cecli/coders/base_coder.py:
  • Removed UsageMeta metaclass, simplified to plain Coder class with instance-level token/cost tracking
  • Fixed edit_format resolution logic (no longer overrides with main_model.edit_format for "code" format)
  • Moved "Local" MCP server disconnection logic from Coder.create() to main.py where coder switching occurs
  • Properly transfers mcp_manager, tui, context_management_enabled, and token/cost accumulators between coders
• cecli/main.py: Disconnects "Local" MCP server when switching from agent to non-agent coder at the switch point rather than during coder creation

Sub-Agent & Delegation Changes

• cecli/coders/agent_coder.py: Removed allow_nested_delegation config option - sub-agents always have Delegate tool excluded
• cecli/coders/sub_agent_coder.py: Always excludes Delegate tool from sub-agents
• cecli/helpers/conversation/integration.py: Sub-agents context block only shown for primary coders (no parent_uuid)
• cecli/commands/spawn_agent.py: Simplified switch hint text
• Documentation: Updated agent-mode.md and subagents.md to reflect removal of allow_nested_delegation

Linter Fixes

• cecli/commands/lint.py: Changed linter.lint() from await to synchronous call (linter is no longer async)
• tests/basic/test_linter.py: Updated tests to match synchronous linter API

Minor Fixes

• cecli/helpers/skills.py: Removed unnecessary Path() wrapping
• cecli/coders/base_coder.py: Changed usage report separator from space to newline for better readability
• cecli/prompts/agent.yml, cecli/prompts/subagent.yml: Fixed typo "allows you too" → "allows you to"
• Removed tests/coders/test_coder_switching.py: Test was fragile and tightly coupled to implementation details that changed

Testing

• All existing tests pass
• Manual testing of Ctrl+C interruption during agent mode processing
• Verified coder switching between agent and non-agent modes
• Verified sub-agent delegation restrictions

[tomjuggler]

Very cool thank you! Ctrl+C after seeing the AI go off the rails in console is an essential feature!