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>

.agents/skills/lib_py/agents/base.py

@@ -208,6 +208,13 @@ class BaseAgentAdapter:
     def input_placeholder(self) -> Optional[str]:
         return None
 
+    @property
+    def input_placeholder_pattern(self) -> Optional[str]:
+        """Regex the input box's first line matches in full while it shows
+        only a placeholder whose text varies (#60); input_placeholder stays
+        the literal lib.sh greps for."""
+        return None
+
     @property
     def input_rule_pattern(self) -> Optional[str]:
         return None