feat(skills): add forge-tdd test-driven development skill

[mgratzer]

Summary

Add a TDD skill to the forge pipeline — currently the biggest functional gap compared to peer skill collections.

Problem / Motivation

Forge has no skill for test-driven development workflows. When implementing features, forge-implement-issue says "test as you go" but doesn't enforce TDD discipline (red-green-refactor cycles, behavior-driven test design, or strategic mocking).

Inspired by mattpocock/skills TDD skill, which includes deep reference materials on testing philosophy and practical patterns.

Proposed Solution

Create forge-tdd with these core concepts:

Red-Green-Refactor with Vertical Slices

• One test at a time, then minimal code to pass — never write all tests first (horizontal slicing anti-pattern)
• Each RED-GREEN cycle is a thin vertical slice
• Only refactor when all tests are GREEN

Behavior-Based Testing

• Tests describe observable behavior through public interfaces, not implementation details
• A good test reads like a specification: "user can checkout with valid cart"
• Tests should survive internal refactors without changes

Mock at System Boundaries Only

• Mock: external APIs, databases, time/randomness, file system
• Don't mock: your own classes, internal collaborators
• Prefer SDK-style interfaces (specific functions per operation) over generic fetchers

Deep Modules (John Ousterhout)

• Small interface + deep implementation = good module design
• Design interfaces for testability: accept dependencies, return results
• Fewer methods = fewer tests = better design signal

Reference Files

Consider 5 supporting reference files:

• references/tests.md — what makes a good test, behavior vs implementation
• references/deep-modules.md — module depth concept and evaluation
• references/interface-design.md — designing for testability
• references/mocking.md — when and how to mock
• references/refactoring.md — candidates and patterns for the refactor phase

Pipeline Fit

Sits after forge-create-issue as an alternative to forge-implement-issue for test-first workflows. Could also be invoked as a companion alongside implement-issue.

Acceptance Criteria

• SKILL.md follows forge skill structure (frontmatter, Input, Process, Output Format, Guidelines, Related Skills, Example Usage)
• Red-green-refactor cycle is enforced step-by-step, not just recommended
• Reference files cover mocking strategy and refactoring patterns
• Cross-skill consistency table updated in coding-guidelines.md
• Pipeline linkage: create-issue → tdd and/or implement-issue → tdd