docs: add customization system documentation and roadmap

[Gaubee]

Summary

Add comprehensive documentation for the workflow/MCP customization system and project roadmap.

New Documentation

1. Prompt Customization (06-configuration/prompt-customization.md)

• Template syntax with configurable PREFIX/SUFFIX (default: {​{KEY}})
• YAML frontmatter for custom syntax and ALIAS mapping
• Built-in placeholders: USER_RULES, CUSTOM_INSTRUCTIONS, DATETIME, CWD
• Priority: user > builtin

2. Workflow Config (06-configuration/workflow-config.md)

• Zod schema per workflow for type-safe configuration
• Tools merge syntax with ! prefix for removal
• Common config: tools, permissionMode, prompts
• Workflow-specific config: openspec (coder), output (research), etc.

3. Roadmap (08-roadmap/index.md)

Near-term (v0.2):

• Project scope: .jixoflow/ directory support
• Roles system: multi-profile configuration spaces

Mid-term (v0.3):

• Plugin system
• Remote configuration
• Team collaboration

Long-term:

• Visual configuration UI
• Multi-runtime support (Node.js, Bun)
• Agent marketplace

Design Notes

• Project scope and user scope are parallel (same dimension)
• Roles are a separate dimension for profile isolation
• Default role is "user" (current user/ directory)
• Roles structure: roles/{name}/{mcps|workflows|user}