refactor: centralize stdout writing

[steipete]

Summary

• add StdoutWriter to centralize serialized stdout writes
• route command/help/RPC/watch output through StdoutWriter
• remove watch-specific stdio flush plumbing and use FileHandle-based line writes
• keep JSONLines.encode helper while delegating line output to unified writer
• reduce test noise by capturing stdout in command/router tests

Validation

• make format
• make lint
• make test

[steipete]

Landed via temp rebase onto main.

• Gate: make lint && make test && make build
• Land commit: afabfc8
• Merge commit: 1b27e3b

Thanks @steipete!