feat(tui): composer soft-wrap, alt+backspace, bracketed paste, borderless input

- editorLines now soft-wraps long logical lines at the box width (caret tracked
  across wrapped rows) instead of overflowing/truncating
- alt+backspace deletes the word before the cursor (deleteWordBack)
- handle tea.PasteMsg so bracketed paste (Ctrl+V / Ctrl+Shift+V / right-click)
  drops clipboard text into the focused composer; was silently dropped
- render the composer (in-session + launcher) borderless: top rule + flush text,
  no side borders or trailing padding, so a terminal drag-copy yields just the
  typed text instead of box chrome and whitespace

apps/tui-go/internal/app/input_test.go

@@ -5,6 +5,53 @@ import (
 	"testing"
 )
 
+// A bracketed paste inserts at the cursor in the main composer (insert mode, no overlay).
+func TestHandlePasteIntoComposer(t *testing.T) {
+	m := NewModel(nil)
+	m.editMode, m.overlay = ModeInsert, OverlayNone
+	m.inputBuffer, m.inputCursor = "ab", 1
+	m = m.handlePaste("XY").(Model)
+	if m.inputBuffer != "aXYb" || m.inputCursor != 3 {
+		t.Fatalf("composer paste = (%q,%d), want (%q,3)", m.inputBuffer, m.inputCursor, "aXYb")
+	}
+}
+
+// A paste while steering appends to the steer note, not the input buffer.
+func TestHandlePasteWhileSteering(t *testing.T) {
+	m := NewModel(nil)
+	m.steering = true
+	m.steerBuffer = "go "
+	m = m.handlePaste("now").(Model)
+	if m.steerBuffer != "go now" || m.inputBuffer != "" {
+		t.Fatalf("steer paste = steer:%q input:%q", m.steerBuffer, m.inputBuffer)
+	}
+}
+
+// Alt+Backspace deletes the word (plus trailing spaces) left of the cursor.
+func TestDeleteWordBack(t *testing.T) {
+	cases := []struct {
+		buf     string
+		cur     int
+		wantBuf string
+		wantCur int
+	}{
+		{"foo bar", 7, "foo ", 4},
+		{"foo bar  ", 9, "foo ", 4},
+		{"foo bar baz", 7, "foo  baz", 4}, // cursor mid-string
+		{"one", 3, "", 0},
+		{"", 0, "", 0},
+	}
+	for _, c := range cases {
+		m := NewModel(nil)
+		m.inputBuffer, m.inputCursor = c.buf, c.cur
+		m.deleteWordBack()
+		if m.inputBuffer != c.wantBuf || m.inputCursor != c.wantCur {
+			t.Errorf("deleteWordBack(%q,%d) = (%q,%d), want (%q,%d)",
+				c.buf, c.cur, m.inputBuffer, m.inputCursor, c.wantBuf, c.wantCur)
+		}
+	}
+}
+
 // F-006: a multi-line paste must collapse to a single display line so the fixed-height
 // input box can't overflow and leave a smeared/stale region.
 func TestFlattenForDisplay(t *testing.T) {