fix(opencode): the 2.x home placeholder is not typed input, cut example included (Issue #60, Linux item 1)

An adapter can now name its placeholder as a pattern the box's first line
matches in full (input_placeholder_pattern); input_placeholder stays the
literal lib.sh greps for. opencode: `Ask anything… "<example>"`, the closing
quote optional because a box under ~65 columns cuts the example. Fixtures:
the 140-column herdr capture and a 60-column one with the cut example.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fixtures/panes/opencode_idle_2x_home_narrow.txt

@@ -0,0 +1,31 @@
+# captured 2026-10-18 from an idle opencode 2.0.32 home screen (no message sent), input box empty, width 60: tmux 60x30 `capture-pane -p`; the example is cut without its closing quote (cwd line masked)
+
+
+
+
+
+
+
+
+
+
+                                            ▄
+           █▀▀█ █▀▀█ █▀▀█ █▀▀▄ █▀▀▀ █▀▀█ █▀▀█ █▀▀█
+           █  █ █  █ █▀▀▀ █  █ █    █  █ █  █ █▀▀▀
+           ▀▀▀▀ █▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀
+
+
+  ┃
+  ┃  Ask anything… "What is the tech stack of this proje
+  ┃
+  ┃  Build · Nemotron 3.5 Super VL Free OpenCode
+  ╹▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
+  /…/<worktree>:<branch>  shift+tab agents  ctrl+p commands
+
+
+
+
+
+
+
+