Fix Tower startup checks and shellper cleanup

[timeleft--]

Summary

Fixes #1030.

• Make afx tower start wait for /api/status readiness by default, with --no-wait preserving fire-and-forget startup.
• Make afx tower stop clean shellpers scoped to Tower's socket dir by default, with --preserve-shellpers retaining restart-preserving behavior.
• Add SessionManager.killScopedShellpers() for explicit operator cleanup while keeping killOrphanedShellpers() conservative for startup reconciliation.
• Include shellper stderr log context when startup info JSON is missing or invalid, without dumping full env JSON.

Validation

• pnpm --filter @cluesmith/codev test --run src/agent-farm/__tests__/tower-command.test.ts src/terminal/__tests__/session-manager.test.ts -t "tower command lifecycle options|kills scoped shellpers|killScopedShellpers|surfaces shellper stderr"
• pnpm --filter @cluesmith/codev test
• pnpm --filter @cluesmith/codev build

Notes

Required external consult was attempted before PR creation. Gemini review was blocked by approval policy because it would disclose uncommitted code to a third-party model without explicit user approval. Codex consult failed before review due to local config: unknown variant default, expected fast or flex in service_tier.