fix(cli): validate surface add before session lookup

[benvinegar]

Summary

• validate sideshow surface add has at least one surface flag before resolving/creating a session
• resolve long-poll comment waits promptly when the client disconnects, avoiding lingering 60s waits in watch-style clients
• expand CLI tests for trace publishing/syncing, top-level help/error handling, surface validation, and guide fallback behavior
• isolate CLI test env from inherited SIDESHOW_* variables and harden watch test cleanup

Coverage impact

• Overall line coverage: 93.94% → 95.03%
• bin/sideshow.js line coverage: 81.72% → 87.29%
• bin/sideshow.js branch coverage: 51.26% → 61.83%

Validation

• node --test --test-name-pattern 'watch streams' test/cli.test.ts
• node --test test/cli.test.ts
• npm test
• npm run typecheck
• npm run lint
• npm run format:check
• npm run changeset:status -- --since=origin/main

Review

• Ran fresh-context reviewer subagents twice.
• Addressed initial review findings around misleading surface-add coverage, inherited env leakage, watch long-poll cleanup, and trace-sync quiet-mode assertions.
• Final re-review found no blockers or major issues.

This PR description was generated by Pi using OpenAI GPT-5 Codex