ci(depguard): enforce architecture boundaries in lint and CI

[kangeunchan]

Enhancement Description

Enforce architecture boundaries in CI with depguard to prevent forbidden dependency directions.

Summary

This PR adds depguard-based architectural checks and documentation so boundary violations are caught automatically during lint/CI.

Previously, boundary enforcement relied heavily on manual review:

• forbidden imports could regress between reviews
• architecture checks were not consistently automated
• remediation guidance was not centralized

Now lint enforces boundary rules and documents the expected dependency direction.

What Changed

1) Enabled depguard architecture rules

Updated:

• .golangci.yml

Rules enforce boundary constraints for application and ports packages.

2) Added staged allowlist for known legacy violations

• scoped allowlist keeps rollout practical
• new violations are still blocked by CI

3) Added architecture boundary documentation

Added:

• docs/architecture-boundaries.md

Documentation includes:

• enforced boundaries
• rationale
• remediation approach

4) Tests

Validation is lint/CI-focused for this change.

Why This Is Needed

This is an architecture governance improvement.
Automated boundary checks reduce review burden and prevent dependency-direction regressions.

Behavior Notes / Regression Impact

No runtime behavior change.
CI/lint behavior becomes stricter for forbidden imports.

Validation Performed

• golangci-lint run --enable-only depguard ./...
• go test ./...