Summary

Add a "smart paste" feature to the links module that automatically converts clipboard URLs into markdown links with fetched page titles.

• Async: Non-blocking fetch using vim.system() (no UI freeze)
• Configurable: links.smart_paste.enabled option (disabled by default, opt-in)
• Fallback: Prompts user for title when fetch fails; canceling replaces placeholder with raw URL
• Trigger: <Plug>(MarkdownPlusSmartPaste) mapping (default: <leader>mp)

How it works

1. User copies a URL to clipboard
2. User triggers <leader>mp in a markdown buffer
3. Placeholder [⏳ Loading...](url) appears immediately
4. Title is fetched asynchronously via curl
5. Placeholder updates to [Page Title](url)

Configuration

require("markdown-plus").setup({
  links = {
    smart_paste = {
      enabled = true,   -- Enable smart paste (default: false)
      timeout = 5,      -- Fetch timeout in seconds (default: 5)
    },
  },
})

Requirements

• curl must be installed
• Neovim 0.10+ (for vim.system() async API)

Health check (:checkhealth markdown-plus) verifies these requirements when smart paste is enabled.

Changes

• Add links.smart_paste config types and validation
• Add lua/markdown-plus/links/smart_paste.lua module
• Add <Plug>(MarkdownPlusSmartPaste) keymap
• Add health check for curl and Neovim version
• Add comprehensive tests for helper functions
• Add vimdoc documentation

Test plan

• Run make check - all tests pass
• Manual testing with various URLs (GitHub, Google, YouTube, etc.)
• Test failure case (invalid URL prompts for title)
• Test cancel behavior (replaces with raw URL)
• Test disabled state (shows warning)
• Test health check reports curl status

Closes #190