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/commands.go

@@ -58,6 +58,10 @@ type tagsLoadedMsg struct {
 	tags []db.TagCount
 }
 
+type railTagsLoadedMsg struct {
+	tags []db.TagCount
+}
+
 type statusClearMsg struct{ seq int }
 
 type editorFinishedMsg struct {
@@ -181,6 +185,16 @@ func loadTags(store *db.Store) tea.Cmd {
 	}
 }
 
+func loadRailTags(store *db.Store) tea.Cmd {
+	return func() tea.Msg {
+		tags, err := store.ListTags(false)
+		if err != nil {
+			return errMsg{err}
+		}
+		return railTagsLoadedMsg{tags}
+	}
+}
+
 func editInEditor(store *db.Store, e *db.Entity) tea.Cmd {
 	editorEnv := os.Getenv("EDITOR")
 	if editorEnv == "" {