feat(tui): add link picker and navigation history

Press [ in detail view to open link picker showing all [[links]]
in the current entry. Enter follows a link, resolving by title
then body substring. Navigation history stack enables esc to pop
back through followed links before returning to list.

Adds Store.ResolveLink() for non-transactional link resolution
from the TUI layer.

internal/tui/help.go

@@ -60,6 +60,8 @@ func renderHelp(width, height int) string {
 			{"!", "toggle pin"},
 			{"r", "run checklist"},
 			{"f", "fill template"},
+			{"[", "follow [[link]]"},
+			{"esc", "back (pops link history)"},
 		}},
 		{"Stumble", [][2]string{
 			{"n / →", "skip to next"},