feat: stream deployment logs with --follow flag

[omattsson]

Summary

• Adds --follow / -f flag to deploy, stop, clean, rollback, and logs commands
• Connects to the backend WebSocket (/ws), subscribes to the instance log channel, and streams deployment.log lines to stdout in real-time
• Exits on terminal status (running, stopped, error, draft) with appropriate error if deployment failed
• Graceful shutdown on Ctrl+C via signal.NotifyContext

Closes #51

Implementation

• New StreamDeploymentLogs client method in cli/pkg/client/websocket.go
• Sends a subscribe message after connecting (required -- the hub uses BroadcastToInstance for log lines, which only reaches subscribed clients)
• New WS types (WSMessage, WSDeploymentLog, WSDeploymentStatus, StreamResult) in cli/pkg/types/types.go
• followLogs helper in cli/cmd/stack.go wraps the client call with signal handling

Test plan

• 11 WebSocket client tests: success flow, error status, instance filtering, context cancellation, JWT auth, API key auth, malformed messages, draft terminal, non-terminal status ignored, connection error, bad scheme
• URL conversion tests (http to ws, https to wss, trailing slash, unsupported scheme)
• All existing tests pass (go test ./...)

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