[refactor] Standardize error aggregation and wrapping with multierr

[kangeunchan]

Enhancement Description

Standardize error aggregation and wrapping patterns using go.uber.org/multierr and consistent error-chain semantics.
Scope type: Error handling consistency and developer ergonomics

Background

Error handling varies across the codebase. Custom multi-error aggregation does not support Go 1.20+ Unwrap() []error semantics, reducing usefulness of errors.Is/As and weakening debugging.

Scope

Replace custom MultiError with multierr.Append

• Files: internal/** (where custom MultiError exists, e.g., docker package)
• Files: go.mod (already depends on multierr; confirm versions)

Ensure custom errors support chaining consistently

• Implement Unwrap() and/or Is() consistently where required
• Prefer fmt.Errorf(... %w ...) for wrapping at boundaries

Add context to operational errors

• Include paths/parameters where meaningful
• Avoid generic “failed” without actionable context

Document guidelines

• Files: CONTRIBUTING.md (or docs/dev/*)

Non-Goals

• Rewriting all errors in the repository in one pass
• Introducing new error frameworks or logging libraries
• Changing user-facing error text wholesale without reason

Risks and Open Questions

• Some error messages are relied upon by scripts; changes must be scoped and noted
• Aggregation semantics must preserve ordering and readability
• Need to avoid leaking sensitive data in error contexts

Validation Plan

Unit and Integration Checks

• go test ./... for affected packages
• Unit tests demonstrating errors.Is/As works through aggregates
• Static checks for removed custom MultiError usage (grep)

End-to-End Checks

• Smoke critical workflows to ensure errors remain actionable
• Validate multi-error outputs remain readable in CLI logs

Evidence Required in Issue Updates

• Before/after example showing errors.Is/As works for aggregated errors
• Grep evidence showing custom MultiError removed
• Snippets of improved error messages with context

Acceptance Criteria

• Custom MultiError usage removed in favor of multierr
• errors.Is/As works reliably for aggregated errors
• Error wrapping guidelines documented
• Operational errors include actionable context

Deliverables

• PR replacing custom MultiError and updating call sites
• PR documenting error guidelines (if separate)

[kangeunchan]

pull request: #129