refactor(up): up command agent result error reconciliation

[skevetter]

Summary

Centralizes how the agent-forwarded structured error is reconciled with the transport error in the up path:

• Adds config.Result.Err() (nil-safe) and ErrNoAgentResult, so the result.Error != "" handling lives in one place instead of being duplicated across up.go and agent.go.
• Splits executeDevsyUp into validateUpResult (result/err reconciliation) and resolveWorkdir (workspace-folder resolution), dropping the function below the cyclomatic-complexity threshold.
• Adds a unit test for Result.Err().

No behavior change.

Context

Extracted from #603 per review feedback to keep that PR focused on the devsy ci feature. This is a prerequisite refactor and targets main directly.

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📥 Commits

Reviewing files that changed from the base of the PR and between 789ddd8 and d3a2ef0.

📒 Files selected for processing (4)

• cmd/workspace/up/agent.go
• cmd/workspace/up/up.go
• pkg/devcontainer/config/result.go
• pkg/devcontainer/config/result_test.go

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