feat(tui): stumble mode — resurface stale entries card by card

Card-by-card walkthrough of entries untouched for 30+ days.
Prevents write-mostly decay by bringing old entries back to attention.

- S from list triggers stumble, loads entries where modified_at < 30d
- Single-card view with markdown body, glyph, tags, age indicator
- Actions: n skip, d dismiss, ! pin, p promote, m absorb, esc exit
- Progress indicator: stumble [3/12]
- After promote/absorb from stumble, returns to deck (not list)
- "All caught up" screen when deck exhausted
- DB: add ModifiedBefore to ListParams, modified_at sort column

internal/tui/help.go

@@ -41,6 +41,7 @@ func renderHelp(width, height int) string {
 			{"#", "filter by tag"},
 			{"m", "absorb (merge into target)"},
 			{"p", "promote to card"},
+			{"S", "stumble (resurface stale entries)"},
 		}},
 		{"Detail View", [][2]string{
 			{"p", "promote to card"},
@@ -51,6 +52,14 @@ func renderHelp(width, height int) string {
 			{"r", "run checklist"},
 			{"f", "fill template"},
 		}},
+		{"Stumble", [][2]string{
+			{"n / →", "skip to next"},
+			{"d", "dismiss (soft delete)"},
+			{"!", "pin"},
+			{"p", "promote to card"},
+			{"m", "absorb"},
+			{"esc", "exit"},
+		}},
 		{"Run Mode", [][2]string{
 			{"j/k", "move between steps"},
 			{"space", "toggle step"},