feat(extensions): Add picker extensions for telescope, fzf-lua, and snacks.nvim

[mhiro2]

Summary

• Add extensions API to push picker results directly onto the peekstack stack
  • Support telescope, fzf-lua, and snacks.nvim picker integrations
  • Each extension provides push_file, push_grep, push_lsp_references, and actions.push for custom
    configurations

Changes

• 0c438c6 : feat(extensions): add common push_entry helper
  • Implement push_entry function to convert picker entries to PeekstackLocation and push to stack
  • Handle nil entry and missing filename edge cases gracefully
• 9eddc16 : feat(extensions): add telescope extension
  • Register telescope extension with push_file, push_grep, push_lsp_references commands
  • Provide actions.push for custom telescope mappings
• 58473c2 : feat(extensions): add fzf-lua integration
  • Implement fzf-lua extension with file, grep, and LSP reference pickers
  • Provide reusable actions.push for custom fzf-lua configurations
• 8a50685 : feat(extensions): add snacks.nvim integration
  • Implement snacks.nvim extension with picker integrations
  • Handle snacks-specific 0-based column conversion and relative path resolution
• 2876603 : feat(extensions): expose extensions API in init
  • Add peekstack.extensions proxy table for direct access to extension modules
• 7a93d6f : test(extensions): add push_entry unit tests
  • Test entry normalization, edge cases, and snacks.actions.push behavior
• 8dbfe52 : docs(extensions): add extensions section to README and vimdoc
  • Document usage examples for all three picker extensions