Add maintainability guardrails and architecture KPIs

[JanSmrcka]

Problem

Architecture drift is hard to detect early without explicit guardrails and metrics.

Scope

• Add CI guardrail for oversized core files (starting with internal/ui/model.go threshold)
• Track monthly KPIs:
  • model.go LOC
  • ignored-error count in internal/ui/*
  • subprocess timeout coverage
  • mocked UI test ratio
  • regression count post-merge

Acceptance Criteria

• CI fails when guardrail threshold exceeded
• KPI collection method documented in repo docs
• First KPI baseline captured

Why

Quantified signals keep maintainability work consistent, not reactive.