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/statusbar.go

@@ -87,6 +87,8 @@ func contextHints(m model) []hint {
 	switch m.focus {
 	case focusCapture:
 		return []hint{{"enter", "submit"}, {"esc", "browse"}, {"?…", "search"}, {"-", "todo"}, {"@", "event"}}
+	case focusTagRail:
+		return []hint{{"j/k", "nav"}, {"enter", "filter"}, {"ctrl+b", "hide"}, {"tab", "list"}, {"esc", "list"}}
 	case focusDetail:
 		if m.splitDetail {
 			return []hint{{"h", "list"}, {"c", "copy"}, {"e", "edit"}, {"p", "promote"}, {"!", "pin"}, {"tab", "capture"}}