refactor(agent-presets,web): copy-only preset authoring with a path to the files

The web YAML editor is gone. agentPreset.write (arbitrary composition
text) became agentPreset.copy { from, agentPreset, name? }: a host-side
whole-directory copy of ids the host resolves itself — symlinks
dereferenced, modes re-tightened to owner-only with owner-execute kept,
metadata rewritten to keep the source's description but never its name or
roster order. No composition text or path crosses the wire in either
authoring direction, and the entryListSchema/!!js concern dissolves with
assertComposition itself.

The settings section becomes: a read-only viewer over shipped
compositions, a copy dialog (id + optional display name) as the only
create entry, delete for custom rows, and a location action leading into
the preset's own files — agentPreset.openDocument { agentPreset } resolves
the directory host-side and opens it natively, or answers
{ opened: false, path } for the row to show as text where the deployment
has no desktop. agentPreset.list reports hasDocument beside authorable;
the gateway's nativeOpen config pins the capability where
canOpenNativePath platform detection would mislead. The privileged set is
now read/copy/openDocument/remove.

With files as the only composition editor, standing mounts grew
stamp-keyed generations: ensureStanding compares the composition file's
mtime+size and starts the next generation for later sessions, while every
joined session keeps the generation it runs on.

New keyless web lane (agent-preset-authoring, overlay pins
nativeOpen: false so goldens render one branch on every platform) drives
view/copy/reveal/delete end to end; the real-composition CLI e2e switches
to copy semantics.

packages/host/apiproxy/src/native-path-opener.ts

@@ -152,6 +152,25 @@ async function openNativePathWithIntent(
   throw new Error(`native path opener is unsupported on ${platform}`)
 }
 
+/**
+ * Whether {@link openNativePath} plausibly reaches a desktop on this host.
+ *
+ * macOS and Windows always carry a desktop opener; Linux does when it is WSL
+ * (the Windows desktop takes the path) or a display server is announced.
+ * A headless or containerised Linux host answers false, which is what lets a
+ * surface show a path as text instead of offering a button that would spawn
+ * `xdg-open` into nothing.
+ * @param internals - platform and environment seam for deterministic tests.
+ * @returns true when handing a path to the native opener can work at all.
+ */
+export function canOpenNativePath(internals: PathOpenerInternals = {}): boolean {
+  const platform = internals.platform ?? process.platform
+  if (platform === 'darwin' || platform === 'win32') return true
+  if (platform !== 'linux') return false
+  const env = internals.env ?? process.env
+  return isWsl(internals) || present(env.DISPLAY) || present(env.WAYLAND_DISPLAY)
+}
+
 /**
  * Open a filesystem path with the operating system's default application, or
  * with the default browser when the path names a document a browser renders.