os-deps: a dependency declares who has to be able to run it

The probe asked the owner's login shell where a binary was, and the install reported that as a fact
about the machine. For a confined plugin the work happens as the member's own Linux user, in their own
home, where the owner's ~/.local/bin is neither on PATH nor reliably reachable.

On alpha it worked — because /home/pastilhas happens to be mode 751, world-traversable, so an
absolute path resolved. At 750, the more usual default, every member would have lost every binary with
nothing reporting it: the install would still have said satisfied, having only ever asked the owner.
Nobody chose 751, and that is the part worth fixing.

`scope: 'owner' | 'shared'` on an osDependency, defaulting to owner, an unrecognised value being a
manifest error rather than a silent fall back to the permissive one.

Shared changes three things. It is searched for on system directories only — asking the owner's shell
would find the copy in the owner's home and call it satisfied, the exact false pass this exists to
stop. Reachability is checked as world-executable AND every parent traversable to the root, because a
binary at 755 inside a home at 700 is unreachable and stat on the file alone says it is fine. And a
release binary goes to /usr/local/bin through sudo -n install, staged in the owner's space first so
the download itself never runs as root.

An unreachable shared dependency gets its own state and refuses the install ahead of every other
check, because no package manager fixes it: the binary is installed and works, and what is wrong is
where it lives — reinstalling would report success and change nothing.

Not a security boundary, and docs/plugin-dependency-scope.md says so: anything in /usr/local/bin is
runnable by any account with a shell, grant or no grant. That is already true of ffmpeg. The plugin's
data is gated; its executable is not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -438,6 +438,10 @@ explaining why it was safe.
   mounted and what it knows, the URL-vs-channel split for panel-to-panel communication, and the
   persistence key families. Read before building a panel app. Its defect list is
   `docs/workspace-panel-todo.md`.
+- `docs/plugin-dependency-scope.md` — a plugin declares WHO must be able to run each host binary. Why the
+  owner's login shell is the wrong question for a `confined` plugin, the two-part reachability check
+  (world-executable AND every parent traversable), and why members' binaries on alpha worked only because
+  one home directory happened to be mode 751
 - `docs/plugin-route-authorization.md` — **design, not built**: making a plugin's settings routes
   owner-only while the plugin itself stays grantable. What the proxy should pass (`isOwner` and a level,
   never the role name), why declaring owner-only paths in the manifest beats trusting every plugin to