feat: offer backend CLI install on missing binary (entrypoints) + shared backend_available/ensure_backend helpers

- Standalone my* wrappers pass offer_install=True so a missing backend CLI
  triggers the interactive 'Install now? [y/N]' prompt (was a bare not-found).
- Add backend_available()/ensure_backend() to expose the backend registry
  (binary/env_bin/install_cmd) for xiaohe init / ensure-deps to reuse.

myagents/entrypoints.py

@@ -2,11 +2,14 @@
 
 from myagents.launcher import build_cli
 
-claude_cli = build_cli("claude", prog_name="myclaude")
-kimi_cli = build_cli("kimi", prog_name="mykimi")
-codex_cli = build_cli("codex", prog_name="mycodex")
-hermes_cli = build_cli("hermes", prog_name="myhermes")
-cursor_cli = build_cli("cursor", prog_name="mycursor")
+# offer_install=True so a missing backend CLI (e.g. `claude`) triggers the
+# interactive "Install now? [y/N]" prompt instead of a bare not-found error.
+# Backends without an install_cmd (hermes/cursor) fall back to the manual msg.
+claude_cli = build_cli("claude", prog_name="myclaude", offer_install=True)
+kimi_cli = build_cli("kimi", prog_name="mykimi", offer_install=True)
+codex_cli = build_cli("codex", prog_name="mycodex", offer_install=True)
+hermes_cli = build_cli("hermes", prog_name="myhermes", offer_install=True)
+cursor_cli = build_cli("cursor", prog_name="mycursor", offer_install=True)
 
 
 def _progs():

myagents/launcher.py

@@ -256,6 +256,28 @@ def _offer_install(config: dict) -> str | None:
     return _resolve_backend_binary(config)
 
 
+def backend_available(backend: str) -> bool:
+    """True if the backend CLI (or its env override) resolves on PATH."""
+    cfg = _BACKENDS.get(backend)
+    return bool(cfg and _resolve_backend_binary(cfg))
+
+
+def ensure_backend(backend: str) -> bool:
+    """Ensure a backend CLI is available, offering to install it when missing.
+
+    Reuses the interactive ``_offer_install`` (prompt on tty, install via the
+    backend's ``install_cmd``). Backends without an installer (e.g. hermes)
+    fall back to the manual not-found message. Returns True if the CLI is
+    available afterwards.
+    """
+    cfg = _BACKENDS.get(backend)
+    if not cfg:
+        return False
+    if _resolve_backend_binary(cfg):
+        return True
+    return _offer_install(cfg) is not None
+
+
 def _launch(
     backend: str,
     chat_cwd: Path,