feat(skills): add forge-improve-architecture skill

[mgratzer]

Summary

Add a skill for proactive architectural friction discovery and refactoring planning.

Problem / Motivation

Forge has no skill for identifying architectural problems before they become bugs. Current workflow is reactive: issues are created when problems are noticed. A proactive skill would explore the codebase organically, surface friction, and propose targeted refactors.

Inspired by mattpocock/skills improve-codebase-architecture, which combines organic exploration with structured dependency analysis.

Proposed Solution

Create forge-improve-architecture with these core concepts:

Organic Codebase Exploration

• Explore like a new team member — note where understanding requires bouncing between many files
• Surface friction: shallow modules, tightly-coupled seams, untested code, functions extracted just for testability

Dependency Categories Framework

Classify each friction point by dependency type, which determines the testing strategy:

1. In-process — pure computation, no I/O → merge and test directly
2. Local-substitutable — has test stand-ins (e.g., SQLite for Postgres) → deepen with local sub
3. Remote but owned (Ports & Adapters) — own services across network → define port, inject adapters
4. True external (Mock) — third-party services → mock at boundary

Parallel Sub-Agent Design Exploration

• Spawn 3+ sub-agents with radically different design constraints
• Each proposes a different interface for the identified friction point
• Compare on: simplicity, general-purpose vs specialized, depth, ease of correct use

"Replace, Don't Layer" Testing

• Delete old tests on shallow modules once boundary tests exist
• Write new tests at the deepened module's interface boundary
• Assert on observable outcomes through public interface

Pipeline Fit

Standalone skill invoked ad-hoc — not part of the linear workflow. Can feed into forge-create-issue when a refactor is identified.

Acceptance Criteria

• SKILL.md follows forge skill structure
• Organic exploration step with concrete friction indicators
• Dependency categories framework with testing strategy per category
• Parallel sub-agent design step for interface alternatives
• Output: GitHub issue RFC with proposed interface, dependency strategy, testing strategy
• Cross-skill consistency table updated