feat: Add cross-platform detection (Claude Code + OpenCode support)

[Steffen025]

Summary

Adds platform detection module enabling SpecFlow to work on both Claude Code and OpenCode platforms without hardcoded paths.

Reference: This addresses the platform detection feature discussed in #8 (SpecFirst Integration Proposal), which @jcfischer welcomed for contribution.

What's Included

packages/specflow/src/lib/platform.ts (172 lines)

Function	Purpose
detectPlatform()	Detect Claude Code vs OpenCode via env vars or directory structure
getRootDir()	Get platform root directory (~/.claude or ~/.opencode)
getPlatformInfo()	Complete platform info including standard directories
resolvePath()	Resolve relative paths to absolute based on platform
isOpenCode() / isClaudeCode()	Platform check helpers

packages/specflow/tests/lib/platform.test.ts (303 lines)

• 28 tests covering all functions
• Environment variable scenarios
• Path resolution edge cases
• All tests passing

Detection Priority

1. PAI_DIR environment variable (explicit override)
2. OPENCODE_DIR environment variable (OpenCode specific)
3. Directory structure detection (~/.claude vs ~/.opencode)
4. Error if neither found

Usage Example

import { getRootDir, isOpenCode } from "./lib/platform";

// Instead of hardcoded:
// const envPath = `${homedir()}/.claude/.env`;

// Platform-agnostic:
const envPath = `${getRootDir()}/.env`;

// Or check platform:
if (isOpenCode()) {
  // OpenCode-specific logic
}

Testing

cd packages/specflow
bun test tests/lib/platform.test.ts
# ✓ 28 pass, 0 fail

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Contributed by @Steffen025 + Jeremy (OpenCode/Claude Opus)