feat: add .claude/commands/ compatibility for command discovery

[SeanZoR]

Summary

Adds support for Claude Code compatible command paths, enabling seamless migration for users coming from Claude Code.

• .claude/commands/ (project-level)
• ~/.claude/commands/ (global)

OpenCode native paths take precedence over Claude-compatible paths.

Fixes #6985

Changes

• config.ts: Added getClaudeDirectories() helper and integrated Claude command loading before OpenCode directories
• config.test.ts: Added 3 tests for Claude command discovery, precedence, and nested paths
• commands.mdx: Updated docs to list Claude-compatible paths with precedence note