feat(tui): collapsible tag rail with ambient tag awareness

Persistent left panel showing tags with counts. Provides ambient
awareness of tag landscape without requiring a modal.

- New tagRailModel in tagrail.go: tag list with cursor, scroll, counts
- Rail visible at >=100 cols width, 18% width (min 16 chars)
- ctrl+b toggles rail visibility
- focusTagRail added to focus cycle: capture → tags → list → detail
- j/k navigates, enter filters/unfilters by tag
- Active filter tag highlighted bold in rail
- Tags refresh after entity create/delete/absorb
- Rail auto-hides on narrow terminals, # modal still works as fallback
- Width allocation accounts for rail in split and non-split layouts

internal/tui/styles.go

@@ -129,4 +129,18 @@ var (
 
 	hintDescStyle = lipgloss.NewStyle().
 			Foreground(dim)
+
+	railHeaderStyle = lipgloss.NewStyle().
+			Bold(true).
+			Foreground(dim)
+
+	railTagStyle = lipgloss.NewStyle().
+			Foreground(lipgloss.AdaptiveColor{Light: "#43BF6D", Dark: "#73F59F"})
+
+	railActiveTagStyle = lipgloss.NewStyle().
+				Foreground(lipgloss.AdaptiveColor{Light: "#43BF6D", Dark: "#73F59F"}).
+				Bold(true)
+
+	railCountStyle = lipgloss.NewStyle().
+			Foreground(dim)
 )