loop: add reload_runtime tool and auto-resume on /reload

[yuanhaorannnnnn]

When running a loop that modifies extension files, the agent previously had no way to trigger reload. After a manual /reload, the loop state was restored but the loop did not resume automatically.

This adds:

• reload_runtime tool: LLM-callable, queues /reload-runtime as followUp
• reload-runtime command: executes ctx.reload() (only available in CommandContext)
• Auto-resume: on session_start with reason="reload" and active loop, automatically triggers the next loop prompt

The LLM can now modify extensions during a loop, call reload_runtime, and the loop automatically continues with the new code loaded.

Flow

Agent modifies .ts extension file
  -> calls reload_runtime tool
  -> followUp queues "/reload-runtime"
  -> current turn completes, agent idle
  -> Command handler calls ctx.reload()
  -> session_shutdown -> session_start (reason: reload)
  -> restoreLoopState() recovers loop state
  -> triggerLoopPrompt() auto-resumes loop