feat: configurable paste collapse thresholds (TUI + CLI)

Adds two new config keys:
- paste_collapse_threshold (default: 5) — line count threshold for
  bracketed paste collapse in both TUI and CLI
- paste_collapse_threshold_fallback (default: 0, disabled) — same for
  the fallback heuristic in terminals without bracketed paste support

TUI frontend reads these from config.get full via applyDisplay/patchUiState.
CLI reads from self.config at paste-handling time.

Closes #5626
Related: #5623

ui-tui/src/app/useConfigSync.ts

@@ -142,6 +142,17 @@ const _voiceRecordKeyFromConfig = (cfg: ConfigFullResponse | null): ParsedVoiceR
   return raw ? parseVoiceRecordKey(raw) : DEFAULT_VOICE_RECORD_KEY
 }
 
+const _pasteCollapseLinesFromConfig = (cfg: ConfigFullResponse | null): number => {
+  if (!cfg?.config) return 5
+  const raw = cfg.config.paste_collapse_threshold
+  if (typeof raw === 'number' && Number.isFinite(raw) && raw >= 0) return Math.round(raw)
+  if (typeof raw === 'string') {
+    const n = parseInt(raw, 10)
+    if (Number.isFinite(n) && n >= 0) return n
+  }
+  return 5
+}
+
 /** Fetch ``config.get full`` and fan the result through ``applyDisplay``.
  *
  * Extracted so the mtime-reload path can be exercised by the test
@@ -188,6 +199,7 @@ export const applyDisplay = (
     indicatorStyle: normalizeIndicatorStyle(d.tui_status_indicator),
     inlineDiffs: d.inline_diffs !== false,
     mouseTracking: normalizeMouseTracking(d),
+    pasteCollapseLines: _pasteCollapseLinesFromConfig(cfg),
     sections: resolveSections(d.sections),
     showCost: !!d.show_cost,
     showReasoning: !!d.show_reasoning,