refactor(error): standardize aggregated error handling with multierr

[kangeunchan]

Enhancement Description

Standardize aggregated error handling with go.uber.org/multierr while preserving Go error-chain semantics.

Summary

This PR replaces custom multi-error aggregation in Docker orchestration with multierr.Append and aligns wrapping semantics for better diagnostics.

Previously, custom aggregation reduced consistency:

• error-chain behavior (errors.Is/As) was weaker across aggregated failures
• wrapping style varied by failure path
• rollback/deploy combined failures were harder to inspect consistently

Now error aggregation follows one standard pattern with explicit tests.

What Changed

1) Replaced custom aggregation with multierr

Updated:

• internal/infrastructure/docker/orchestrator.go

Changes:

• removed custom multi-error implementation
• standardized aggregation via multierr.Append

2) Standardized wrapping at failure boundaries

• preserved causal context using %w
• ensured deploy and rollback failures compose consistently

3) Added usage guidance

Added:

• docs/error-handling.md

Guidance covers:

• %w wrapping conventions
• multierr aggregation pattern
• preserving errors.Is/As behavior

4) Tests

Added/updated tests:

• internal/infrastructure/docker/orchestrator_test.go

Why This Is Needed

This is a correctness and maintainability refactor.
Standardized error chains improve debugging, observability, and tooling compatibility.

Behavior Notes / Regression Impact

No intentional user-facing behavior change.
Error surfaces are more consistent while preserving root causes.

Validation Performed

• go test ./internal/infrastructure/docker -count=1
• go test ./...
• golangci-lint run ./internal/infrastructure/docker/...