codeSplitter team threads: hosted runs hear notes while they work #3713

From #3711 (team threads), item 1. #3712 (claude on the desktop hears notes over stream-json stdin) has merged, so this can start off main.

sandbox/server.mjs runs the CLI with stdin ignored and answers once it exits, so a hosted run still only hears a note on its next run.

• Add a /tell route the AgentSandbox Durable Object forwards to the running process's stdin (claude's --input-format stream-json, the same streamMessage execCommand writes).
• Have containerSandbox (src/sandbox.ts) poll the run's ear and POST each note there, returning heard / unsaid the way execCommand does.
• Set listens for hosted runs in execute (src/agent-routes.ts).

Notes already reach the run as @who: text (said in agent-routes.ts), and a listening run's first prompt already carries LISTEN_ASK (harness.ts), so neither changes here.

Part of #3423.