feat: add telescope.nvim integration for session selector

[ushmz]

Summary

• Implement full telescope.nvim session selector with custom picker, previewer, and configuration support
• Add selector factory module to dynamically select selector backend based on ui.selector config option
• Enable runtime switching between native, telescope, and float selectors without code changes

Changes

New Features

• Telescope Session Selector (lua/tabi/ui/selector/telescope.lua)

  • Custom picker displaying session name, note count, date, and branch
  • Buffer previewer showing session details and note summaries with truncation
  • Support for ui.telescope.theme and ui.telescope.layout_config options
  • Graceful fallback with error notification when telescope.nvim is not available

• Selector Factory (lua/tabi/ui/selector/init.lua)

  • Dynamic selector selection based on config.get().ui.selector
  • Fallback to native selector for invalid or nil configuration values

Configuration

require("tabi").setup({
  ui = {
    selector = "telescope", -- 'native', 'telescope', or 'float'
    telescope = {
      theme = "dropdown",  -- optional: telescope theme
      layout_config = {},   -- optional: layout configuration
    },
  },
})

Test plan

• All 30 new tests pass (22 telescope selector + 8 factory tests)
• All 307 total tests pass (no regression)
• Luacheck passes with 0 warnings
• Stylua formatting check passes
• Manual testing: Run :Tabi retrace with ui.selector = "telescope" to verify telescope picker appears

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