feat(skills): add Agent Skills management system

[fdc310]

Implement comprehensive skills management feature inspired by agentskills spec:

Backend:

• Add Skill and SkillPipelineBinding database entities
• Add database migration (dbm018) for skills tables
• Implement SkillManager for skill loading, matching, and resolution
• Implement SkillService for CRUD operations
• Add skills API endpoints for skill and pipeline binding management
• Integrate skill index injection into pipeline preprocessor
• Add skill activation detection in LocalAgentRunner

Frontend:

• Add Skills page with listing, search, and type filter
• Add SkillDetailDialog for create/edit with preview
• Add SkillCard and SkillForm components
• Add skills API methods to BackendClient
• Add skills entry to sidebar navigation
• Add i18n translations (en-US, zh-Hans)

Features:

• Support skill and workflow types
• Sub-skill composition via {{INVOKE_SKILL: name}} syntax
• Progressive disclosure (index in prompt, full instructions on activation)
• Pipeline-specific skill bindings with priority

概述 / Overview

请在此部分填写你实现/解决/优化的内容:
Summary of what you implemented/solved/optimized:

更改前后对比截图 / Screenshots

请在此部分粘贴更改前后对比截图（可以是界面截图、控制台输出、对话截图等）:
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修改前 / Before:

修改后 / After:

检查清单 / Checklist

PR 作者完成 / For PR author

请在方括号间写x以打勾 / Please tick the box with x

• 阅读仓库贡献指引了吗？ / Have you read the contribution guide?
• 与项目所有者沟通过了吗？ / Have you communicated with the project maintainer?
• 我确定已自行测试所作的更改，确保功能符合预期。 / I have tested the changes and ensured they work as expected.

项目维护者完成 / For project maintainer

• 相关 issues 链接了吗？ / Have you linked the related issues?
• 配置项写好了吗？迁移写好了吗？生效了吗？ / Have you written the configuration items? Have you written the migration? Has it taken effect?
• 依赖加到 pyproject.toml 和 core/bootutils/deps.py 了吗 / Have you added the dependencies to pyproject.toml and core/bootutils/deps.py?
• 文档编写了吗？ / Have you written the documentation?