feat(skills): add forge-write-prd skill

[mgratzer]

Summary

Add a PRD writing skill for complex features that need more structure than forge-brainstorm provides.

Problem / Motivation

forge-brainstorm produces a plan summary suitable for straightforward features. For complex, multi-issue features, teams need a full Product Requirements Document with user stories, implementation decisions, testing strategy, and explicit out-of-scope declarations. Currently this requires manual effort outside the forge pipeline.

Inspired by mattpocock/skills write-a-prd, which produces structured PRDs through deep codebase exploration and relentless interviewing.

Proposed Solution

Create forge-write-prd with these core concepts:

Deep Interview with Recommended Answers

Adopt the grill-me pattern already integrated into brainstorm:

• Walk down each branch of the design decision tree
• For each question, provide a recommended answer based on codebase exploration
• If a question can be answered by exploring the codebase, explore instead of asking

Codebase-Grounded Requirements

• Validate every assumption against the actual codebase before including in the PRD
• Reference existing implementations, patterns, and constraints
• Identify "deep module" opportunities (small interface, deep implementation)

Structured PRD Output

## Problem Statement
## Solution
## User Stories
1. As a [actor], I want [action] so that [benefit]
## Implementation Decisions
## Testing Decisions
## Out of Scope
## Further Notes

Pipeline Fit

Sits between brainstorm and create-issue for large features:

• setup → brainstorm → write-prd → create-issue → implement → reflect → address
• brainstorm outputs a rough plan; write-prd deepens it into a full requirements document
• create-issue then breaks the PRD into vertical-slice issues

Acceptance Criteria

• SKILL.md follows forge skill structure
• Interview phase explores codebase before asking, provides recommended answers
• User stories are specific and testable, not generic
• Implementation decisions reference actual code and patterns
• Out-of-scope section is explicit and grounded
• Related Skills links to brainstorm (before) and create-issue (after)
• Workflow order updated in cross-skill consistency table