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/tab_sessions.go

@@ -11,6 +11,7 @@ import (
 	"cburn/internal/tui/components"
 	"cburn/internal/tui/theme"
 
+	"github.com/charmbracelet/bubbles/textinput"
 	"github.com/charmbracelet/lipgloss"
 )
 
@@ -26,6 +27,49 @@ type sessionsState struct {
 	viewMode     int
 	offset       int // scroll offset for the list
 	detailScroll int // scroll offset for the detail pane
+
+	// Search/filter state
+	searching   bool            // true when search input is active
+	searchInput textinput.Model // the search text input
+	searchQuery string          // the applied search filter
+}
+
+// newSearchInput creates a configured text input for session search.
+func newSearchInput() textinput.Model {
+	ti := textinput.New()
+	ti.Placeholder = "search by project, cost, tokens..."
+	ti.CharLimit = 100
+	ti.Width = 40
+	return ti
+}
+
+// filterSessionsBySearch returns sessions matching the search query.
+// Matches against project name and formats cost/tokens for numeric searches.
+func filterSessionsBySearch(sessions []model.SessionStats, query string) []model.SessionStats {
+	if query == "" {
+		return sessions
+	}
+	query = strings.ToLower(query)
+	var result []model.SessionStats
+	for _, s := range sessions {
+		// Match project name
+		if strings.Contains(strings.ToLower(s.Project), query) {
+			result = append(result, s)
+			continue
+		}
+		// Match session ID prefix
+		if strings.Contains(strings.ToLower(s.SessionID), query) {
+			result = append(result, s)
+			continue
+		}
+		// Match cost (e.g., "$0.50" or "0.5")
+		costStr := cli.FormatCost(s.EstimatedCost)
+		if strings.Contains(strings.ToLower(costStr), query) {
+			result = append(result, s)
+			continue
+		}
+	}
+	return result
 }
 
 func (a App) renderSessionsContent(filtered []model.SessionStats, cw, h int) string {