chore(stack): make logs --follow tail through deploy completion

[omattsson]

Summary

stack logs --follow already exists (closed #51). Ensure it:

• Tails through deploy completion (closes stream cleanly when server signals end, not on first idle).
• Respects Ctrl-C without leaking goroutines.
• Reuses the WS auth chain from 7.1 if it migrates from polling to WS.

Files to touch

• cli/cmd/stack.go — logsCmd follow path.

Tests (all three layers required)

• Unit: cli/cmd/stack_test.go + (if migrated to WS) cli/pkg/client/websocket_test.go — terminal-status close path, Ctrl-C path, goleak.VerifyNone(t).
• Integration: extend cli/test/integration/stack_integration_test.go — Cobra in-process logs --follow completes when stub signals end.
• E2E: cli/test/e2e/cli_e2e_test.go — stackctl stack logs <id> --follow against stub returns 0 within timeout.

Acceptance

• Stream closes cleanly when deploy reaches terminal status.
• goleak.VerifyNone(t) passes for the follow command test.
• No behavioural regression for --follow=false path.

Tracks #59