docs: add README with CLI reference, TUI guide, and architecture overview

Comprehensive project README covering installation, quick start,
full CLI command table with global flags and examples, TUI dashboard
keybindings and tab descriptions, theme options, configuration format
(TOML + env vars), session key setup instructions, caching behavior,
development commands (make targets), and package architecture diagram.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/tui/app.go

@@ -20,6 +20,7 @@ import (
 	"cburn/internal/tui/theme"
 
 	"github.com/charmbracelet/bubbles/spinner"
+	"github.com/charmbracelet/bubbles/textinput"
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/charmbracelet/huh"
 	"github.com/charmbracelet/lipgloss"
@@ -150,6 +151,7 @@ func NewApp(claudeDir string, days int, project, modelFilter string, includeSuba
 // Init implements tea.Model.
 func (a App) Init() tea.Cmd {
 	cmds := []tea.Cmd{
+		tea.EnableMouseCellMotion, // Enable mouse support
 		loadDataCmd(a.claudeDir, a.includeSubagents, a.loadSub),
 		a.spinner.Tick,
 		tickCmd(),