feat(mcp): integrate AI agents with the massCode vault - #944

AI agents can now access the massCode vault through an opt-in MCP server using the existing Integration API token. Agents can search and read saved items, create snippets and notes, and create or execute saved HTTP requests.

• Add MCP connection settings and authenticated local access with Host/Origin validation and bounded payloads, responses, and execution time.
• Reuse the HTTP execution pipeline while rejecting local file uploads through MCP; preserve snippet line endings when content is reloaded from Markdown.
• Document setup for Codex and other MCP clients, HTTP execution behavior, and availability in v5.13.
• Add an MCP homepage section, extract the existing homepage sections into components, group Command Palette with workflows, and standardize heading weights.

Validation: 48 MCP tests and 22 related tests passed during implementation and security review. Scoped ESLint and the VitePress production build passed after the homepage changes. The docs build retains an existing CSS nesting warning and uses cached GitHub statistics when the stats refresh is unavailable.